Now that I think about it the Joby S4 is a tiltrotor given all 6 of its propulsion units tilt from vertical to horizontal for flight and have a wing that generates lift just like the V280. So I guess tiltrotors did make the cut for use in UAM ops.

Joby doesn't have rotors, it has propellers, so it's a tiltprop... which is better than the tilt+lift configuration Archer and Wisk are using. However, the use of props instead of rotors does limit the width of the conversion corridor and the agility in VTOL mode. However, since it cannot sustain the power draw necessary for VTOL flight for more than a few minutes, I suppose they consider that an acceptable trade off.

The difference in noise between the 609 and S4 has something to do with the more than 3X difference in gross weight but the 550 fps vs ~800 fps tip speed is also hugely important. Tip speed is the most important parameter in noise generation. The 609 could be quieter, but would require larger blade chord and torque to compensate for lower rotor rpm and that would drive up empty weight or reduce payload. All the eVTOL aircraft I've ever examined have serious weight challenges and the drive for low tip speeds is a contributor (electric motor sizing, like transmissions, are mostly sized by torque).

Its my understanding variable tip speed capability, the number of lifting/thrust units used, and electric propulsion were all required to get the low noise levels required for UAM ops. And what those blades are called is still up for debate considering the FAA calls them props, EASA lift/thrust devices, and NASA prop-rotors.

From what I've seen its the same reason for any new aircraft concept when they are first fielded: the technology needs to mature. For example, as new hybrid power units are released by most turbine OEMs for the e-aviation market, I think some of those performance limits will be reduced as the eVTOL concept matures.