So 1090ES uses messages that are 112 bits maximum (14 bytes) and of that 24 bits (3 bytes) are used for addressing and other bits are used for control functions, leaving very few bits for data. Plus 1090 MHz is subject to saturation as it is used for ADS-B, TIS-B, transponder replies. The transponders are continuously interrogated by ground secondary radars and by TCAS/TAS units. UAT on the other hand uses messages that are 432 bytes of payload (3456 bits), so each UAT message has way more room for transmitting FIS-B data. Any aircraft that has access to a UAT receiver can receive the FIS-B data in the flight levels.
