basically because the ADSB bandwidth is used up.
True, congestion on 1090MHz is a growing problem, with ATCRBS, Mode S, ELS/EHS, TCAS, and MLat all competing for the spectrum, though in this case it was organisational rather than technical issues that stalled the project.
The system BAA was looking at not only did automated AROT/DROT calculations, but would typically capture 50-100 or more data points during a landing rollout, allowing it to produce a deceleration profile (you could even deduce what autobrake setting was being used) and accurately measure exit speed. Obviously this made use of the fact that ADS-B sends velocity as a first-order parameter so you're not dependent on calculating groundspeed averaged over a succession of radar or MLat plots.