Buddha says "While the differences between these species are defined by their birth,the differences between humans are not defined by their birth.In individual human bodies you can’t find such distinctions. The distinctions among humans are spoken of by convention.For name and clan are assigned as mere convention in the world.Arising by mutual agreement,they’re assigned to each individual.For a long time this misconception has prejudiced those who don’t understand.Ignorant, they declare that one is a brahmin by birth.You’re not a brahmin by birth,nor by birth a non-brahmin.You’re a brahmin by your deeds, and by deeds a non-brahmin.(https://suttacentral.net/mn98/en/sujato)
Why cant Hinduism accept Guna Based Flexible Varna system as Buddhism?
Because it goes against Vedas and is impossible to implement. Plus a true Brahmana, Kshatriya, etc. is one by birth and by action.
Chhandogya Upanishad 5.10.7 - Those whose conduct has been good here will shortly get birth such as a Brahmana, a kshatriya, or a vaishya. But those whose conduct has been evil will be born in evil births shortly such as the birth of a dog, or as a pig, or a chandala.
Maitrayani Samhita 1.4.1 - We know not if we are brahmins or non-Brahmins. ...Therefore, when recounting our gotra-pravaras, say that the devas are our ancestors.
Taittiriya Brahmana - 'When a Sudra woman is the mistress of an Arya, she does not seek wealth for prosperity.' Therefore they (the priests) do not bestow royal consecration on the son of a Vaishya woman.