All the major points have been touched on above: interest rates, lending criteria, cycles, jobs/economy, etc.
I'll expand on two other points that I feel have affected the current state of the market (in)directly:
1) Expected returns - those following paper asset returns have heard of the anticipated 6% nominal return over the next couple of years based on current factors (not to mention the ever present and eventual stock market correction(s)). Therefore people, whether individual or institutional, will always gravitate to where higher returns are expected/anticipated. Since the recession there is no shortage of people and examples (point #2 below) of the kinds of returns RE has provided and it's not surprising that everyone wants to get involved.
2) Proliferation of information - I may be just me, but I definitely think the availability and dissemination of "free" information regarding RE investing - websites, forums, podcasts, (e)books, etc. - has grown significantly over the years (thanks interwebz!) and has naturally influenced more people to get involved. I have a lot more friends/colleagues who have suddenly shown interest RE investing in just the last couple of years then ever before.
To go with the point #2, I think there is also more RE "experts" out there in general since it seems like just about everyone who had managed to get involved in RE post-2007/8 will claim to have experienced positive returns. Of course that was the best time to get into RE. Buying at the bottom of the stock market and riding out the recovery doesn't necessarily make you a professional stock picker either ;)
Economic cycles are well...cyclical. Always interesting to see in the future how everyone involved in RE will come out of it after the next downturn/correction/recession.