There are 2 reasons that I can think of. 1) Safety reason - different plug type to wall outlet prevents user from mistakenly plug in a 110V appliance to a 240V wall outlet. Anyway, this is still not a fool proof solution.
2) Business / Marketing strategy - Manufacturer like Sony, prefers to have their own unique style / electrical sockets to prevent user from using other brands accessories with their product - ie: camera with unique rechargeable battery. So, the next time you have a faulty accessory, you need to come back to them to get a replacement. It's most of the time the accessory that is damaged and it's expensive too.