What makes Hillary's PPP numbers so poor compared to the rest?
We don't know that it is poor compared to the rest. She could genuinely be losing some ground. Quinnipiac just had a national poll in which her numbers were unimpressive:
https://uselectionatlas.org/FORUM/index.php?topic=174292.0OTOH, if her numbers are not fading, then PPP is giving us a very interesting electoral map. She's leading Rubio in Kentucky by 6, but only 4 in Virginia and 4 in Colorado. That would be a heck of a reversion to 1990s style electoral maps if that holds up. Could be that with Clinton not having the big comparative advantage among younger voters that Obama has, she doesn't do as well in high population growth states that have trended D under Obama, like CO and VA.
In any case, the trend map that PPP is giving us so far looks like it bears some similarities to the Clinton-Obama differential map from SUSA's 2008 polling:
