UA583 is showing J9 C9 D9 Z7 P0 O0 A0 R0 Y9 B9 M9 E8 U0 H0 Q0 V0 W0 S0 T0 L0 K0 G0 N9 for me right now.
And I can get UA583 inventory down to W when married with INTL segments, e.g. DUBEWR or AMSEWR. INTL fares typically (but not always) allow connecting to p.s.
So there's definitely inventory available. They just didn't want to sell it to you for a low price. Route, POS, etc. can all be factors.