The reason for not allowing HD skins in the later rounds is for two reasons.
1. The point of PBL is to create limitations in the number of colors you can use. Many times these limitations are what make it fun, challenging and produces the best works. Keeping the standard skin resolution is essentially creating another limitation. It would be a lot easier to do dither shading in a HD skin, but harder to achieve that at the standard resolution. So we lean more towards the challenging format.

2. We want to keep things balanced and fair in the polls. 1v1 polls and community voting is at the very heart of PBL. If we allowed HD skins, we may see unfair advantages being created in polls if someone did a a HD skin, and the other person didn't. If both entries were HD, that would be fine. But we can't guarantee it and we don't want to force people to change their format to match their opponent.

Absolutely nothing against HD skins and I wish we could include them in SSPBL, but I can't see a good way to do it without making things unfair. Hope that helps explains our reasons for not allowing them.