The Big Corporations are Phasing out the 2-strokes Because..
There is less money to be made in it. The companies with dominating shares of the market (such as Honda or Yamaha) can throw their clout to influence the markets. As was said earlier, 4-strokes COST more to maintain. More maintenance means more money for dealers (keeping them happy), and forcing consumers to buy more parts for their toys. A majority of those consumers will not maintain them, causing the bikes to break down sooner, which means that they'll probably buy another bike sooner as well. This all equates to more money for the Manufacturer.
It's hard to push a product that dealers know would eventually make them obsolete. I'm sure the dealers lobbied for this phasing out as well.
This is one way a marketer could increase the 'lifetime value' of a customer for their products. And in a corporation where ethical dignity are sometimes set aside in the pursuit of money, it certainly is an option. I feel like I see this in every industry.
Obviously this is a severely abbreviated summary, but hopefully you guys get the gist of what I am trying to say.
-Remorseful Marketer who loves his two-strokes.