Part of the problem is the contest and who wins, but the other part is that the threads are just out of control. Typically 10 members have 50% of the posts in the whole thread.
In the last contest, there were 416 members that posted in the thread. 203 of them (half) had only 1 post. Why is that? Because they post once, see that it's mainly just a mad flurry of non-sensical posts by a small group of members and they don't come back. Is that good for the forum? (for the record, there's currently 150 members in the current contest with only 1 post)
The top 10 posters in the last contest accounted for 45% of the posts in the entire thread. In the current contest, the top 7 posters have over half the posts. The contests have just become a playground for 10 or 15 members to chit chat.
As a result, a very large segment of members don't even come to the contest threads any more. They were started as a means of driving traffic and they've resulted in just the opposite.
Nobody wants to post and then come back an hour later and see that in order to catch up with the discussion they have to read another 500 posts (by 5 people).
In one of the competition threads I saw quite a few members say they will post once to enter the competition but they won't be hanging around because of the 257997467807 posts.
They were active members on the forum so they wasn't just posing just in the competition thread.
For my chit chat I post in threads which are specifically for that purpose. I have great conversations with other people and getting to know them.