In my opinion... Maybe I am wrong. I apologize to say... The forum is dying now-a-day, It appears to be....The fourm code is so bloated, ugly, confused, and it can be so tedious and time consuming for Forum Administrators especially when they have to fighting off againstspammer every day, every week, every month and year. Facebook just became the new kind of forum, Since its user controlled social network, sharing files, and etc, and wouldn't be much complicated to block off spammer from user list.
OR, is it because...
3d car modeling are fading away... More and more 3d cars are available at shop, or shared site. I assure you lot of user would rather buy or use free model, then materialize and render it right away with render engine of their choice. rather than dealing or spending days by days, weeks or year to complete detailed model. polyflow issues is what kill their motivation fast. Not only that, more and more user despite showing work because of less tolerance on negative comments and constructive crits.
More likely....
Everyone got life, attend to school or jobs and have little time to work on their project or visit site (like me).
But yet, don't let that bother you or get you down. I assure you... Users are still around watching.., its just that they just got too lazy to sign on and say 'Hi, its a Nice work'.