The rationale might be that the USAAC was hedging their bets aginst the Boeing P-26 Peashooter. The Gladiator had far superior performance in just about every area.
From Wikipedia:
Gloster Gladiator Performance
Maximum speed: 253 mph (407 km/h, 220 kn) at 14,500 ft (4,420 m)
Cruise speed: 210 mph (340 km/h, 180 kn)
Stall speed: 53 mph (85 km/h, 46 kn)
Endurance: 2 hours
Service ceiling: 32,800 ft (10,000 m)
Rate of climb: 2,300 ft/min (12 m/s)
Time to altitude: 10,000 ft (3,048 m) in 4 minutes 45 seconds
Boeing P-26 Performance
Maximum speed: 234 mph (377 km/h, 203 kn)
Combat range: 360 mi (580 km, 310 nmi)
Ferry range: 635 mi (1,022 km, 552 nmi)
Service ceiling: 27,400 ft (8,400 m)
Rate of climb: 719 ft/min (3.65 m/s)
The Gladiator first flew about 18 months after the Peashooter, so the timeline kind of works. There were American aircraft of the same period that were the equal or even slightly better, but most were used by the US Navy, and others never went beyond the testing stage. The Consolidated P-30 was a fairly good aircraft, and a monoplane, but required a 2-man crew, which the Air Corps wasn't really sold on, and only 60 were built.
