Because nobody expected much sales on SW to offset the loses. But if they see potential revenue on PS+ for several years and a way to lock to PS5 they MAY do it. That is just supposition.
Myself I think they won't drop bellow 199 regular price with limited 149 promotions (maybe, just maybe a firesale at 99 on the HW with some added SW to break even on all).
PS4 & XB1, probably wont go below 149$ in their lifetimes.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071