That is a function of the market price of STEEM. The intent of witnesses who have raised it is to ensure that the "get $X.XX of Steem Power" offer to new users is at least reasonably attractive, and not just a few cents.

It's not something that has come up for me, since I joined back in July... I was under the impression that the power down minimum was based on a multiple of the amount the particular account was seeded with. If it is pegged to the witness parameter, I can see how that would be an issue for very small accounts...

I don't see it as something that would affect most users, though.