Depending on the riding conditions (normally this happens to mountain bikes) and if you do not remove your cranks to do any kind of service a lot of dirt is gathered at the left side of the crank axle (as you can see in the next foto). Because of all this dirt the cranks are stuck and can not pass through the bottom bracket.

In the past I had to punch the axle with a rubber hammer and when the axle was in the bottom bracket I used to carefully punch the chainrings from the inside (on the screws). Whith this tool I just screw it on the non drive side of the cranks and I punch it with a rubber or even better with a metal hammer.

Tip: when I have removed the cranks I always clean the dirt from the axle with a DREMEL (I use a brush which removes only the dirt and rust and no material from the axle)