Though there are more shows in HDTV than ever before, a great deal of programming isn't. The first question is "why not?" The are several reasons why so much programming is SD.
The number one reason a show might be SD is, up until recently, most shows weren't being produced in HD. Even shows that were shot on film (which actually has more available resolution than HD video) were often edited on SD video equipment. This means those shows would have to be re-edited from the raw film stock for HD - a very expensive option. Some shows have a great deal of syndication revenue potential and are worth the expense, others simply aren't. Other shows (such as those produced in the 70's) were shot on video tape that wasn't even as good as SD video now. These shows can never be converted to HD. Since many cable networks are these popular older shows, SD programming is going to be out there for a long time.
Another big reason for all the SD programming is lack of HD equipment. HDTV is still quite new and expensive. This means there simply isn't enough equipment to go around for all the TV events out there. That's why there are still many sporting events shown in SD. You need an HD production truck, cameras, as well as control room and transmission equipment to create an HDTV event. Right now, networks have to pick and choose which events will be HD since there is a shortage of those HD items.
The third reason some shows are SD is the type of equipment needed. Shows that use a lot of miniature or wireless cameras usually have to use SD equipment since HD technology hasn't quite caught up with those devices.
The final reason you see so much SD video is some stations simply have no immediate plans to broadcast HD. Converting a network (or even a small station) to HD can cost millions. Some channels simplimply don't generate the revenue (or have enough viewers) to warrent the expense.
The other question you might be asking is "will everything ever be in HD?" Probably not until all the SD video gear that is in service breaks down and is no longer available for purchase. At that point, stations will have no choice but to upgrade. That is still a long way off.