The beauty of the financial sector, #1 - there's a revenue stream already. You just need to skim a little of it. That's really how virtually all get-rich-big systems work.

Beauty #2 - The scale of existing financial organizations makes them particularly slow and resistant to change. Lots of opportunity for disruption, if you can avoid getting squashed.

Beauty #3 - The sudden reach of mobile devices puts a lot of computing and network power into easy reach of startups.

Between these three things, it is any surprise that the sector is booming?