First of all, the 757 market has been beed from below with the A321 and 738/739 seating only marginally fewer passengers. Since most airlines that require new 180-200 seat aircraft already fly the 737 or A320, why introduce a completely new aircraft to their fleets? Secondly, the airliner market has been terrible for a few years now, and airlines are looking to smaller aircraft to provide higher frquencies instead of larger aircraft to increase capacity with the same frequency.
The bright side of the coin is that the 757 has much better airfield performance than either the 739 or A321, so used 757s will probably sell like hotcakes in the not too distant future (i.e. the rumor about HP
buying ex-National 757s). These lovely birds will be flying for quite some time to come since they still have excellent economics and meet stage 4 noise standards.