Due to his boon. Nothing else. Without the boon, any of the Dev's could've pulverised him to ash.
'Let me become endued with
a thousand arms when I am in the midst of my troops. While, however, I remain
at home let me have, as usual only two arms! Indeed, let combatants, when
engaged in battle, behold me possessed of a thousand arms, observant also of
high vows, let me succeed in subjugating the whole earth by dint of my prowess.
Having acquired the earth righteously, let me sway her with vigilance.There is a fourth boon which, O foremost of regenerate persons, I solicit thee to grant. O
faultless one, in consequence of the disposition to favour me, it behoveth thee to
grant it to me.Dependent that I am on thee, whenever I may happen to go wrong, let the righteous come forth to instruct and set me right! Thus addressed, the Brahmana replied unto the king, saying, 'So let it be!' Even thus were those boons acquired by that king of blazing effulgence.
Let us look at the celestial voice during battle of P”Ram and Bhishm.
Bhishma is one of the foremost of Vasus! O
son, it is fortunate, that thou art still alive! Santanu's son by Ganga--a celebrated
Vasu as he is,--how can he be defeated by thee? Desist, therefore, O Bhargava!
That foremost of the Pandavas, Arjuna, the mighty son of Indra, hath been
ordained by the Self-create to be the slayer of Bhishma!'
Runner Bhishma's stature as a Vasu is signified here rather than his prowess or worth.. Implying that P'Ram cannot beat portion of a Vasu who is lowest among class of gods. Gods and his ancestors said how Bhishma being a Vasu, can he be killed by P”ram.
Same P”Ram who effortlessly clapped Kartavirya Arjun.. SO you can only Imagine what could’ve Happened to Kartavirya had he fought gods without boons. Someone like Indra could fry him to kebab if not for his boon.
Most “Conquerors” of heaven were successful due to the Benevelonce of Gods. Due to the respect they hold for other Dev's who grant boons to these attackers. Indra would've effortlessly Fodderised the Like of Ravana, Kartavirya, Hiranyakashipu, etc… if not for their boons.