It's driven by the amount of material that students need to ingest over a 3 month period. As sciguy noted, this is totally normal in academic settings as well. For instance, OS (15-410) at CMU requires 70+ hours a week. That's a single class.
These are of course completely different animals, but the scale of learning is similar. Getting a solid intuition for not only web applications but programming fundamentals when starting with neither requires serious effort and is a challenge of breadth. 3 months worth of 40 hour weeks will probably not cut it.
Just because CMU does it too doesn't mean that it's reasonable. Students who are stressed out and sleep-deprived aren't going to be learning effectively - there's lots of research that indicates that sleep is necessary for forming long-term memories.[1] Better to split the course into two semesters.
You've touched on an important point. Namely, that bootcamps actually can't very feasibly split or extend their curriculum. It's not a coincidence that most of these schools offer 10-12 week sessions. Moving past that and the logistics of relocation, tuition, expenses & health care become much more difficult to manage.
These are of course completely different animals, but the scale of learning is similar. Getting a solid intuition for not only web applications but programming fundamentals when starting with neither requires serious effort and is a challenge of breadth. 3 months worth of 40 hour weeks will probably not cut it.