I find it hard to believe that Google has a viewing cap on HP. I really can't figure out how they would do that without limiting what HP ranks for across all sub-domains.
I think HP doesn't rise for a number of other factors. Here are 5 reasons why I think HP views stay roughly constant.
1. Although new people join all the time, there are other people leaving all the time. People who thought it was going to be easy to make money here and failed so have decided to quit. More experienced people who's hubs have suffered and are receiving less traffic so they are deleting those hubs here and taking them elsewhere, where they think they will get that traffic again. Lots of experienced hubbers left after the intial Panda release for this reason and because of the way Panda was handled.
2. A lot of the content on HP is old and no longer updated. This could be because it was written by a spammer, or the person who wrote it is no longer active on HP. This would mean when an update comes along and these hubs are hit and no longer get the previous traffic levels no one is fixing them to get those views again.
3. Hubs get flagged for new violations, such as unrelated links etc (which were acceptable before), and are unpublished until the writer fixes the offending material. This leads to experienced hubbers sometimes scratching their head and then leaving HP when they feel they have been treated unfairly in the matter. Older hubbers in the 2nd category won't get fixed and will cease to be re-published, again resulting in HP losing views.
4. It is harder to rank your hub now than it was a few years ago. This means higher competition areas are harder to rank for.
5. Although new people join on a daily basis, most of them are inexperienced in how to garner views. They commonly don't understand how to rank, how to choose keywords, what niches to write in, how to write their articles to increase their chances of ranking and the list goes on. I really think this is why HP has created the Apprenticeship program.
I hope that makes sense and was helpful.