Sex crimes are pretty hard to prove and convict. It is especially hard against wealthy and powerful people that folks have a vested interest in not seeing them convicted. A lot of very wealthy, famous and powerful people attended Singer's parties and it would not be a good for them if word got out about sex crimes taking place at those parties. We already know convicted sex offenders attended Singer's parties and were a part of his circle which doesn't look good combined with the fact Singer also liked hanging out with some barely legal looking dudes.
Cosby and Weinstein escaped justice for decades.
Cosby and Weinstein only got brought down when they were both far less powerful. Cosby is eighty, going blind and already burned a lot of support in the African American community with his overly judgmental egotistical act he had been doing the last twenty years.
Weinstein's company was making loses and on the brink of collapse before the scandals about him broke. The entertainment industry had no real need to keep covering for those guys but they do for Singer who is still a director and producer on a number of successful projects.