Because they aren't retiring the 757 fleet for a good few years??
Whilst the 757 aren't due for retirement, the 739 is not a good seller. It is a compromised design and suffers from poor runway performance and higher risk of tailstrike. The A321 is outselling it by a margin. If they go that route and decide to replace the 757 they would probably be better with a small sub fleet of A321 operationally, despite the additional training/spares comonallity costs. Just my opinion though.