Why is it called the ''hammer'' in curling

It might be because of it is great power in deciding the outcome of any given end.

Yes, as the first answerer implied, since having the last stone to throw in the end is very important No scoring in any end is made until the hammer stone (last stone) comes to a stop. It can remove a lot of opponent stones and sneak into the button to score. Also, a miss on the hammer can be disastrous, as Saskatchewan knows in the close loss to Alberta, where what could have been a 11-6 Sask game with one end left turned into a 7-7 tie with a missed hammer.