First of all, the 6522 has some things (parallel ports and timers) that a UART simply doesn't have at all.
A UART will normally generate some programmable number of start bits, stop bits, and possibly parity bits. The number of data bits will typically also be programmable (e.g., on an 8250, anywhere from 5 to 8 data bits). Insertion of the start and stop bits is what makes it "asychronous"--that is, adding the start/stop bits allows asynchronous transmission. The start and stop bits are used to synchronize transmission, even though the transmitter and receiver don't (otherwise) share any clocking. The level of configuration is what makes it "universal"--that is, a single chip was supposed to be compatible with essentially any serial standard from an ancient Teletype ASR32, using 5-bit Baudot codes at 110 bits per second, all the way up to a 28800 bps modem using 8 data bits, no parity, 1 start bit and one stop bit (and faster than that, up to some upper limit on speed--115200 bps for a lot of designs, but newer ones go up to a few megabits per second in some cases).
The 6522 wasn't programmable to that degree. It just took 8 bits of data in a register, and pulsed them out on a serial line (or read 8 bits on a serial line and put them into a register). No start bits. No stop bits. No parity. Only 8 bits, no more, no less. Without the start/stop bits, it doesn't support asynchronous transmission, and with programmable parameters, it's not "universal". So I guess if we started from "UART" and removed the parts that don't apply to a 6522, we're left with an "RT".