Yes, Bali Maharaj was a noble man and also have favors of Brahmanas and noble persons. But Aditi, the mother of Demi-gods, had completed the `Payo-Vrata` to please Lord Vishnu. So, Lord Vishnu had given her the boon to protect her sons. This is described in [Śrīmad-Bhāgavatam, Canto 8, Chapter 17][1].

**SB 8.17.16**

>prāyo ’dhunā te ’sura-yūtha-nāthā

>apāraṇīyā iti devi me matiḥ

>yat te ’nukūleśvara-vipra-guptā

>na vikramas tatra sukhaṁ dadāti

>O mother of the demigods, in My opinion almost all the chiefs of the demons are now unconquerable, for they are being protected by brāhmaṇas, whom the Supreme Lord always favors. Thus the use of power against them now will not at all be a source of happiness.

**SB 8.17.17**

>athāpy upāyo mama devi cintyaḥ

>santoṣitasya vrata-caryayā te

>mamārcanaṁ nārhati gantum anyathā

>śraddhānurūpaṁ phala-hetukatvāt

>Yet because I have been satisfied by the activities of your vow, O goddess Aditi, I must find some means to favor you, for worship of Me never goes in vain but certainly gives the desired result according to what one deserves.

**SB 8.17.18**

>tvayārcitaś cāham apatya-guptaye

>payo-vratenānuguṇaṁ samīḍitaḥ

>svāṁśena putratvam upetya te sutān

>goptāsmi mārīca-tapasy adhiṣṭhitaḥ

>**You have prayed to Me and properly worshiped Me by performing the great payo-vrata ceremony for the sake of protecting your sons. Because of Kaśyapa Muni’s austerities, I shall agree to become your son and thus protect your other sons**.

[1]: http://www.vedabase.com/en/sb/8/17