Because ICEs are overrated. Many systems can't be debugged by single stepping (think servos or anything with physical hardware being controlled). ICEs rarely work well—every single one I've ever used was completely unreliable and required lots of fiddling to make it work. And then the next day you had to start the whole fiddling process over again. In the end they aren't very productive except for very specific types of bugs (they are invaluable in the very beginning of a project when you are bringing up a board). Once everything is generally up and running I find them to be pretty useless.