I don't know. There use to be a lot of polypro baselayers, and up until not that long ago, the U.S. military used polypro quite extensively, but they eventually dropped it because it has a low melting point compared to some other fabrics. In combat, that is a bad trait. That's why poly-cotton blends are used extensively now. On the related civilian side, it's very easy to ruin a polypro garment by accidentally drying it in the dryer on high for too long. Much harder to damage polyester that way.
Amongst outdoors people, polypro baselayers were quite popular for awhile, but they became notorious for becoming extremely smelly and potentially in quite a short period of time. That is why i advocate wearing thin Merino underneath polypro, to reduce that odor buildup. The merino will act like a bit of a filter layer for the things that cause bacteria growth, like dead skin flakes, oils, etc That stuff is easy to wash out of Merino because it is hydrophilic whereas it's hard to wash out of polypro because it super hydrophobic--that gunk and bacteria build up gets stuck in-between the fibers leading to bad funk fast.
Polypro is not as tough/strong as polyester, so chances are a polyester fleece will last a bit longer than polypro.
These perhaps are some reasons why polypro is not dominating the fleece market, but ultimately, i don't know. Also, things in life and in the market tend to go in cycles. For a long time, wool was relatively unpopular and the newer polyester sportswear all the rage with backpackers etc, but now look at wool and how popular it's become again. People are fickle and most tend to be followers to an extent, markets are fickle, most people and the market is generally easy to manipulate.
Personally, i'm extremely interested in getting one of those polypro fleeces and it doesn't matter to me one whit or not, if it's not mainstream and popular.