Well, on the contrary the question maybe reframed, and asked why the present age is Kaliyuga, when great Saints & Yogis like Sri Ramakrishna Paramhamsa, Swami Vivekananda, Yogananda, etc. have walked on this plane?
The answer to why the first Yuga is named Satayuga, and the last Yuga is named Kaliyuga, is simple:-
Because the scriptures says so.
A few evil people, or a few godly people in a certain age (Yuga), just cannot be the sole criterion to determine the Yuga.
Let's see the description of Satyuga in the scriptures, and we shall get the clear answer.
Chapter 27, Tīrtha-māhātmya, Nāgara-khaṇḍa, Skanda Purāṇa
Verses 10 to 29
Kṛta Yuga is said to consist of seventeen lakh twenty-eight thousand years, when Janārdana remains white in colour, and Dharma has
all its four feet intact.
(Then) the earth is full (in all respects). The people are devoid of lust and anger, fear and hatred. All people are long-lived with
quiescent souls and full control over the sense-organs.
They are as tall as five Tālas (Palms). They are brilliant and well-informed. Men’s childhood extends to sixteen thousand years.
Thereafter the youth extends to thirty-two thousand years. It is only
thereafter that old age befalls men slowly. The ultimate life span is
one lakh years.
In certain cases it extends even beyond that. All living beings, animals, birds and beasts speak the divine language. They are not
mutually inimical. Serpents sport about along with mongooses, cats
along with mice, deer with lions, and owls with crows. Even without
being cultivated, the earth (though not ploughed) produces much
vegetation consisting of cereals, pulses, barleys etc. They are tasty
and able to accord strength and nourishment. The trees bear fruits in
all the seasons with excellent flowers and fruits, good leaves etc.
They are devoid of thorns and they resemble Kalpa trees.
Cows yield the desired tasty milk at all times. The butter and ghee produced from it is enormous. No woman becomes a widow there, nor an
unfortunate wretch. There is no Kākavandhyā (a woman with but one
child) there bereft of sons. There is no woman without virtue (all
women are virtuous). Birth and death befall men in due order. A father
never sees death of his son. People who die do not become ghosts. No
one is compelled to stay in Naraka. No one suffers pain due to
sickness. All Brāhmaṇas are well-versed in Vedānta. They regularly
study their Vedic Mantras. They are delighted in expounding the Vedas
and well-versed in the knowledge of Brahman.
Kṣatriyas make one of them the king with great devotion and loyalty. They enjoy the earth righteously following his behests.
Vaiśyas are engaged in the various activities befitting the Vaiśya community such as breeding of cattle, buying and selling of
O most excellent one among Suras, Śūdras are extremely faithful. They do not do anything except service unto Brāhmaṇas and twice-born
No one is base-born there nor is there anyone of mixed caste. No impure one and no one of the fifth caste is to be seen there on the
Men there perform Yajña (Fire-worship), Yajana (Yajña on behalf of others), Dāna, Vrata, Niyama etc. and pilgrimage without any
expectations of personal benefit thereof. O Thousand-eyed One, of this
sort is the first Yuga named Kṛtayuga that is conducive to the
happiness of all the worlds
So basis the above mentioned divine characteristics found amongst the beings and other forms of existences as a whole, in the age of Satayuga, the first of the Yugas is known as Kṛta Yuga or Satayuga.
Just because few demons or asuras or daityas (/danavas, rakshasa), etc. exists in Satayuga doesn't make it Kaliyuga, just like the presence of godly saints in the Kaliyuga doesn't make it Satayuga.