Basic Apache is free, and has been quite well wrung out over the last several years in terms of efficient operation, ease of administration, and being relatively bulletproof in both execution and security. You can get commercial support for Apache based systems, even from IBM (they based their WebSphere product on it and contributed code to the Apache project.)
Linux......... that's another question....... more importantly, why Linux? Are you asking about Linux vs. Mac vs. Windows? Or vs. Unix vs. *BSD?
The shortest answer is they're cheap and they work.
ChiXJeff touched on the good points. To compare, price out a Windows server, and then ASP, and any Databases needed to run these forums, the hosting for Windows based... and management... can't do "un-mananged" windows... as Windows PS's are virus/worm bait..
BUT... it comes down to Chevy VS Ford. We sll know Chevy kicks ass.... but there are still those that, for some reason, continue to fondle ford.