Couple of reasons I can think of.
1. Upper caste/Brahmin guilt (an imported idea mimicking liberal white guilt). They being the earlier ones to get into British india services and then in Nehruvian India, have internalized secular values. Notice this virus more common in BongBrahms + TamBrahms, two places where EIC made early forays. There are other groups like Punjabi Khatris, KPs and Sindhis who are notorious for their dhimmitude which is probably some sort of Stockholm syndrome manifesting itself.
2. A lot of them have a subconscious sense of elite entitlement that hardened into power-by-birth. Its based on birth-class ideas of superiority-inferiority that detaches elite from Hindu commons (non-elite UC included). So much so that they are willing to collaborate with invader/ideology/non-Hindu entities promising profits.
3. Some of them probably think that they can join the core of post-Hindu political establishment. Conversion to Islam worked out great for Rajputs, Jatts, Khatris, Baniyas and Brahmins of Pakistan. They are the ruling elite over there.
4. Influence of crypto-Abrahamic cults like Brahmo Samaj which made Bengal a fertile ground for anti-Hindu Communism. A lot of bong bhadralok left commies came from a wealthy feudal, Brahmo background. Similar to Punjabi Hindu Khatri commies from Arya Samaj background, another Hindu hating crypto-Abrahamic cult.
Here's Raja Ram Mohan Roy :-
Brahmo samaj principles :-
For Arya Samaj, just read Satyarth Prakash and the garbage it spews against other Hindu sects, Tantra, Sikhism, Buddhism, Shaiva, Bhakti, Jains etc.
The problem with cults like Arya Samaj and Brahmo samaj is that they arose as a reaction to Abrahamisms in very troubled times. Hindus were trying to defend their polytheist/idolatrous religion from vicious attacks by monotheists. So these cults resorted to 'de-heathenize' their followers (defensive) by falsely claiming that idol worship/polytheism are 'evils' even according to Hindu scriptures, to get more credibility in the eyes of their monotheist opponents. And these kind of thought patterns that were cultivated over generations eventually led to people rejecting the Abrahamic style Hindu copies in favor of the original Abrahamisms or spin offs like Marxism.
That's why I tell people to stay away from rascal Babas who say we should shun ritualism/idol worship in favor of pure spiritualism/advaita etc. It just paves the way for Abrahamization in future generations. Abrahamisms have no problem digesting "spiritualism", they can't however digest paganism in its outwardly ritualistic form. Check out the evangelization thread and you'll see how this obsession with "rationalism" (pseudo), "esoteric spiritualism" , zeal to "reform the evils", aversion to ritualism etc led to erosion of Graeco-Roman polytheism and eventual Christianization of Roman empire.