IMO its not just one thing but rather a combination..
#Too many spamming producers
#most rappers have their "go to" guys for beats
#most people surfing the web are looking for free sh*t...
#Most people on forums are looking to bs or challenge opinions...
#People tend to not see any value in things they can access for free....
etc..
The truth is if you are looking to sell beats, you should have you're beats posted up on sites where people go to buy em..(eg:
rapinstrumental.net,
soundclick.com, etc..) Its way easier to connect and collaborate with people via a marketplace versus a forum.
$.02