Many reasons for lots of legacy software that just gets updates and refreshes. Someone mentioned the small size of the market, which I think is the main reason. Originally, these programs were designed and written by EEs themselves, who weren't really experts at SW. Today's software has moved towards user friendliness and onto the web. The EE tools are mostly based on older windows platforms or Sun workstations that ran primitive x-windows environments which looked like windows 3.1.

The best stuff is usually pretty expensive. Although Altium is making lower cost and free versions available with their CircuitMaker series.