It isn't as easy as you make it sound. The pledge amounts are only numbers and a descriptor in the database. The Kickstarter database doesn't know that $25 is an art print book or the $222 is seven books. Never mind that Rich hacked together his own workaround to make updateable pledges and even he doesn't know what every pledge means.
It also depends on how they set up their database; a change from one-to-one to one-to-many relationship can add another table and change the database design.
I do think Kickstarter should look toward making this an integral part of their pledging system. I think Rich had a really smart and innovative way of encouraging people to add onto their pledge and to follow the progress of the drive, instead of just firing and forgetting.
Technically, you're right, it's not a hard change when done at the beginning but with the pledge drive ongoing, I wouldn't mess with something that's working as intended and mostly fine.