wiki says "Brahmo Samaj (Bengali: ব্রাহ্ম সমাজ Bramho Shômaj) is a Hindu reform movement. It is the societal component of Brahmoism, a monotheistic reformist movement of the Hindu religion that appeared during the Bengal Renaissance. It is practised today mainly as the Adi Dharm after its eclipse in Bengal consequent to the exit of the Tattwabodini Sabha from its ranks in 1839"
There are a lot of problems with Brahmo philosophy. They don't adhere to the real Hindu philosophy and reject the Vedas. That's why after the advent of Ramakrishna Paramhansa, who realigned the people to the correct form of substantial Hinduism, the formal Brahmo society was dissolved. Individuals might still adhere, but there's no prominence. It was dissolved mainly because Hinduism is a substantial religion, with something to offer to those who adhere to it. Brahmo on the other hand was simply an aimless version of Hinduism mixed with Christian universalism.
[To the moderators]: This answer doesn't need scriptural references, so I took the liberty to place this as an answer