Only rate one answer per thread helpful?

Why is it that now we can only rate one answer per thread helpful.

The idea is...you're rating the best answer that was most helpful for the person out of all the ones that have been posted.

but I developed the habbit of rating every answer that sticks out at all to me.

what if...
I were to rate an answer helpful, but then later a better answer was added.

I usually wait like 3 days and then go back and rate the first 3 that I though were the most helpful.