Numerous reasons.
1) Most MM's such as NITE, ETMM, HDSN, ETC.. are capital markets, meaning other investment institutions, funds, or even wealthier investors can hire these capital markets to perform their trades for them, position them at the best possible price per share.
Example, if you have a bigger investment institution or individual wanting to take a position and they seek out a capital market, would that institution or individual want the capital market to get them a position at .23? or .19 or lower?
Of course, the lower the entry the more profit margins and realized gains one will see. So the lower the better, remember the term, buy low, sell high.
2.) MM are known as Market Makers, so.. one of their duties is to see that the market is in line and does not get out of hand. If all MM's did was just stocks run higher, then there would be no point in the stock market itself. MM's make the market by controlling PPS in unison and in a controlled fashion moving PPS up (when its time), sideways, or at times moving PPS down (currently case for PMAH).
3.) MM's play off (majority of the time) charts / technicals. Meaning, if there is a key resistance point (50 day SMA for PMAH) than the MM's will not let the PPS break above so easily, most likely if there isn't enough volume, PPS will be drawn lower and resistance will hold. "IF" PPS can break above key resistance points, than the stock will run, as more outside investors will be drawn to the stock, seeing that it has broken above a key resistance point.
Same applies to a support level, when a stock is falling, MM's will line up the BID or referred to as "BID STACKING" at a key support level in order to hold that support level. However, "IF" PPS were to break below that key support level, than price would fall rapidly as more investors would sell seeing it broke key support.
So as MM's or Market Makers, they do have a duty and job to adhere to, and must take all of the above and much more into consideration before acting upon. Its not as easy as, just letting a stock run higher and profiting.
Most times PPS movements are calculated and seen within the charts and technicals, however one must learn how to accurately read these charts / technicals to get a better understanding of where PPS might be headed. Of course just my thoughts.