And yet, they discontinue a lot of games too. It would seem that once sales dip below a certain point, it doesn't make sense for a company to keep a product on the market, so they don't. This isn't some big conspiracy. If WotC thought they caould make money by selling old versions of D&D, they probably would (one could argue that, as a publicly traded company and a duty to their stockholders to maximize profits, they would almost be required to). But they don't. From this we can come to a couple of different conclusions:
1. WotC has determined that offering old versions of D&D for sale is not profitable.
2. WotC is staffed by people who hate older versions of D&D with an irrational passion.
3. WotC is run by people too incompetent to know how to make money.
I don't think the correct answer is (2) or (3).
Also, while you can purchase older games, many times those older games have had their rules modified. The game "Life" for example, now has substantially different rules from the version I had when I was a kid. However, there are not two versions of "Life" on the market. There is not "Classic Life" and "Updated Life". There is only one version sold. For players of D&D, the situation is fairly similar.