I think that the success of the photo contest thread is due to the fact that an entry can be generated pretty quickly. I don't know how long everyone takes to go out and photograph, but I know that it could be done in under a minute if one were so inclined. I think that is the reason for the success of the photo competition over the 2-D competition, because it is much easier to take a photo and host it than to create a drawing or photoshop image or painting or what have you. If you look at words, it is mostly short poems with really no formal structure. I just think that people don't have the time and/or motivation to spend a long time generating original content. Creating art can be very time consuming and a lot of work.
That isn't to say that it couldn't succeed. I know that during one discussion of the possible creation of an Arts forum I said that I would contribute and was in support of it, but then things got complicated in life and I didn't really have the means or motivation to do so and I really didn't

. But things changed and I now am in a position where I can/would like to contribute again. People contribute when they can (I am actually waiting for the next 2-D design competition to start because I would like to contribute to that. I would also like to see perhaps a short film competition). I think this is apparent in the photo competition. The pool of contenders is constantly changing. I know some people seem to contribute every round, but lots of people will throw in a submission only when they have the time/inclination. I think that you have to entice people to compete sometimes. For instance, the "Photographer's Choice" round had a huge number of entries, because it allowed more people to contribute. It caught the attention of more people and thus drew greater participation. The photo contest, in virtue of its long tenure, has also gained some momentum, and drawn more participation because of this. I think that these things lend some credibility to the "if you build it they will come" argument, because it shows that once an arts thread has been established in the proper way, and is structured in such a was as to be very inclusive, it can be successful.
As it stands, the Arts threads related to creating original content have not been as successful because they never really gained the momentum that they needed to be successful, which could be due to the fact that they are sandwiched into SO and do not have a good "user base" if you will. These are both problems that could be solved by creating a place that attracts people that would be interested in them in the first place. It's why you create a community arts center rather than telling all the artists to use the school gymnasium for their art shows.
While it has been observed that an arts forum may not succeed by virtue of its existence alone, this doesn't preclude it from succeeding if it is able to find a good base of users; if it is able to hit its target audience. I think that to point to the state of individual art-related threads as being indicative of a general disinterest in the arts and thus proof of the unavoidable failure of an arts forum is to jump to conclusions. Rather than being indicative of a lack of support for arts related threads, it could point to a lack of a proper "forum" (pun intended) for arts related threads to achieve their potential.
Another thing that fuels the perception that there are not many arts related threads is (as has been mentioned) that many of them are scattered amongst the other forums. There are threads in PD, SO, the Lounge, CD, even S&T, that would fit in an Arts forum. They just diffuse over the rest of the board.
One of the keys to making any such forum a success would be to make it known that it exists, and good leadership (moderation i guess). The photo contest probably would have died a long time ago if SA wasn't committed to running it.
Anyway, I have lots of ideas, but I've already written enough.
To sum it up, I guess I am saying that I think it
could be a success, it just needs the right ingredients to get the momentum it needs to continually draw new users, as well as keep those who would contribute regularly, to generate new original artistic content and discussion. I think there is a good chance that they are out there, it is just hard to rally around a bunch of random threads throughout the board and a handful of threads that lose traffic and visibility by being dumped into a forum that doesn't get a great deal of traffic in the first place.
I want to write more but I will stop now.
p.s. no offense or anything to anyone who runs SO or any of the threads I have mentioned.
p.p.s. add "IMO" to every conclusion drawn here. I don't have any data or anything to back it up, it is just conclusions I have drawn from my own observation.