Bilal was sold as a cool soul artist at the begining of his career and his music was first directed to people that didn't want to hear some weird experimentation shit and some crazy ass live performances. IMO that was the first problem with Bilal, he could't be tamed and threw a lot of people off when he started showing who he really was to the public he was first marketed to. He could have had some more attention (not as much as D'angelo but some more) but he wouldn't/couldn't compromise and act like his "first public" would've liked him to act. Also it feels to me that he had missed or wasted all the opportunity that were given to him to get attention (TV appareances, ect) and that's a real problem, he doesn't know how to adapt or he doesn't want to adapt to different audiences.

Then came the whole "Love For Sale" drama, the album was a good departure from his debut, it was left but not too weird, challenging but still accessible to his former fan base but it got leaked and Bilal stoped doing music for a while. He did capitalized on the leak after since he toured a lot on the strenght of that unreleased album but he kind of missed his momentum with this shelved album : he lost the rights on theses songs, didn't make any money on them, didn't gain any "official industry" attention.

When he officially came back with Airtight's Revenge he definitely lost a lot of the attention that he could've gotten with Love For Sale. Imo this album was too far from his former fan base tastes (the original neo soul lover, OKP type of listener, ect) so they just stoped caring about him. Of course the album was praised by the critics but this is not really the attention we're talking about here. The Love For Sale and Airtight's Revenge episode are why he lost a lot of attention. And he kind of said "fuck it!" after this episode and got even more weird than he already was (musically).

D'angelo created a movement, stayed coherent with his former sound, succeed in challenging his fan base but not too much. He was always more tamed and smooth than Bilal and he was there before him. Add to that, that Bilal's vocals are weird, that his sense of what sounds good or not is weird, that some of his songs are weird, that the man himself is weird and you have your answer.