Registered Members get 100 Conversations!
Staff & Donators get Unlimited Conversations!
As for WHY, these messages DO take up database space. We have to put space limit caps somewhere. If we ever hit a point where we need to mass-delete a bunch of stuff to save space, mass deleting public forum threads is easy. We CAN'T mass delete Private Messages because we don't know what content is in there and what is important to the member. :(
So members have to be responsible for keeping their Inbox tidy.