Because there's no reason why they can't if they're switching to MPEG-4. It shouldn't cost that much more to arrange the feeds in a smart manner across their rented transponders to allow for maximum bitrate to each or use DVB-S2.
I'm sure the extra transponder space costs money. And don't forget, just because there's a long-term deal in place, doesn't mean that if both SES and PBS want to change it, it cannot be changed. I've heard that AMC21 is full, and it wouldn't surprise me if PBS wanted to shrink their bills and SES wanted to free up another transponder, so the two would mutually benefit from reducing the transponder space used by PBS.
- Trip