Good question.

My first answer would be that while both technologies operate over broadband, they vary quite extensively in nature. IPTV involves content creation, royalties, digital rights management, and publication to name a few major differences. Companies must spend a lot of time negotiating these agreements and they can be quite costly. You'll see telephone companies and larger companies reaching agreements with major content labels for distribution. Other companies will crop up offering STBs and IPTV over your standard broadband like you suggest. Many of them will offer custom programming and/or video blogging rather than your typical television lineups. There are already some that exist...just look at KyLinTV, ITVN, or Akimbo.

Another point is that we are talking about broadcasting video and not just voice. It's taken many years to get to the point where VoIP is a viable option. Look for IPTV to explode over the next 5-10 years as broadband speeds continually increase and consumers buy into it.