Why? Look at Erieye. The current generation has GaN TRMs, with a much higher power density than the original GaAs TRMs. Looks the same externally. Also has different processors behind them, different screens for the operators, etc. SAAB offers existing customers upgrades all the way up to the latest version. Design the architecture right, so you can take out an outdated piece of kit & slot in a new one, & a system like can be akin to grandad's axe.I have heard elsewhere that the USAF have been hanging out for AESA technology to mature further before making the next leap (something about S-band T/R modules of a particular power density or some such). Hearsay at this point I confess but would explain the persistence with Sentry.
Design the physical architecture right & it becomes a fairly minor job to integrate it on different aircraft, too: Erieye's been operated on a Fairchild Metro for development, & in air force service on the SAAB 340, SAAB 2000, Embraer EMB-145 & now the Bombardier Global 6000.
That shouldn't be beyond the abilities of the US companies in that line of business. After all, they're much bigger than SAAB.