As told by another user in his answer, Incompetence is not at all the reason for Krishna.
There are two key reasons behind this
Krishna did not choose to kill Jarasandha to make his task easier and
Krishna obeyed the command of Brahma that Jarasandha has to be Killed by Bhima only
Now, I am providing the scriptural statements for both reasons from the Mahabharata and Śrīmad-Bhāgavatam.
#1: Krishna did not choose to kill Jarasandha to make his task easier.
It is well-known that the intention behind Krishna avatar is to ease the earth by removing her burden (i.e., danavas, asuras). It can be clarified from the following statements of Mahabharata
"Vaisampayana continued, 'Having said so unto the Earth, O king, the
divine Brahman bade her farewell. And the Creator then commanded all
the gods saying, 'To ease the Earth of her burden, go ye and have your
births in her according to your respective parts and seek ye strife
(with the Asuras already born there)'. And the Creator of all,
summoning also all the tribes of the Gandharvas and the Apsaras, spake
unto them these words of deep import, 'Go ye and be born amongst men
according to your respective parts in forms that ye like.'
"And all the gods with Indra, on hearing these words of the Lord of
the celestials--words that were true, desirable under the
circumstances, and fraught with benefit,--accepted them. And they all
having resolved to come down on earth in their respected parts, then
went to Narayana, the slayer of all foes, at Vaikunth--the one who has
the discus and the mace in his hands, who is clad in purple, who is of
great splendour, who hath the lotus on his navel, who is the slayer of
the foes of the gods, who is of eyes looking down upon his wide chest
(in yoga attitude), who is the lord of the Prajapati himself, the
sovereign of all the gods, of mighty strength, who hath the mark of
the auspicious whirl on his breast, who is the mover of every one's
faculties and who is adored by all the gods. Him, Indra the most
exalted of persons, addressed, saying, "Be incarnate." And Hari
replied,--'Let it be.'"
[Section 64, Adivansavatarana Parva, Adi Parva, The Mahabharata]
In order to accomplish his purpose, Krishna use to leave Jarasandha after destroying all the army of Jarasandha. You can notice that Krishna stopped Balarama from killing Jarasandha in the same chapter of Śrīmad-Bhāgavatam.
Although Lord Kṛṣṇa, the Supreme Personality of Godhead, is the
original cause of this world, when He descended to the earth He played
the role of a human being. Thus when He saw Jarāsandha’s assembled
army surrounding His city like a great ocean overflowing its shores,
and when He saw how this army was striking fear into His subjects,
the Lord considered what His suitable response should be according to the time, place and specific purpose of His current incarnation.
[The Supreme Lord thought:] Since it is such a burden on the earth, I
will destroy Jarāsandha’s army, consisting of akṣauhiṇīs of foot
soldiers, horses, chariots and elephants, which the King of Magadha
has assembled from all subservient kings and brought together here.
But Jarāsandha himself should not be killed, since in the future he
will certainly assemble another army. This is the purpose of My
present incarnation — to relieve the earth of its burden, protect the
pious and kill the impious.
[5-9, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
Krishna implemented the same policy for seventeen times i.e., Krishna use to destroy the total army of Jarasandha and then leaving Jarasandha alone.
Seventeen times the King of Magadha met defeat in this very way. And
yet throughout these defeats he fought on with his akṣauhiṇī divisions
against the forces of the Yadu dynasty who were protected by Śrī
Kṛṣṇa. By the power of Lord Kṛṣṇa, the Vṛṣṇis would invariably
annihilate all of Jarāsandha’s forces, and when all his soldiers had
been killed, the King, released by his enemies, would again go away.
[41-42, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
But Krishna decided to shift Yadus, the eighteenth time, for the benefit of inhabitants only.
Just as the eighteenth battle was about to take place, a barbarian
warrior named Kālayavana, sent by Nārada, appeared on the battlefield.
Arriving at Mathurā, this Yavana laid siege to the city with thirty
million barbarian soldiers. He had never found a human rival worth
fighting, but he had heard that the Vṛṣṇis were his equals. When Lord
Kṛṣṇa and Lord Saṅkarṣaṇa saw Kālayavana, Kṛṣṇa thought about the
situation and said, “Ah, a great danger now threatens the Yadus from
two sides. This Yavana is besieging us already, and the mighty King of
Magadha will soon arrive here, if not today then tomorrow or the next
day. If powerful Jarāsandha comes while We two are busy fighting
Kālayavana, Jarāsandha may kill Our relatives or else take them away
to his capital. Therefore We will immediately construct a fortress
that no human force can penetrate. Let Us settle our family members
there and then kill the barbarian king.”
[43-48, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
Thus, Krishna did not runaway from any war. He took safety measure for Yadus and then killed Kālayavana. It was the decision by Krishna due to the participation of Kalayavana.
#2: Krishna obeyed the command of Brahma that Jarasandha has to be Killed by Bhima only
It is the command of Brahma that Jarasandha is not destined to die in the hands of Bhima only. Krishna obeyed the command of Brahma and hence refrained himself in doing so. It can be clarified from the following statements of Mahabharata
Vaisampayana said,--"Having spoken thus, and desiring to fight with
those heroes of terrible achievements, king Jarasandha ordered (his
son) Sahadeva to be installed on the throne. Then, O bull of the
Bharata race, the king, on the eve of battle, thought of his two
generals Kausika and Chitrasena. These two, O king, were formerly
called by everybody in the world of men by the respectful appellations
of Hansa and Dimvaka. And, O monarch, that tiger among men, the lord
Sauri ever devoted to truth, the slayer of Madhu, the younger brother
of Haladhara, the foremost of all persons having their senses under
complete control, keeping in view the command of Brahma and
remembering that the ruler of Magadha was destined to be slain in
battle by Bhima and not by the descendant of Madhu (Yadavas), desired
not to slay himself king Jarasandha, that foremost of all men endued
with strength, that hero possessed of the prowess of a tiger, that
warrior of terrible valour."
[Section 22, Jarasandhta-badha Parva, Sabha Parva, The Mahabharata]