I think scutum started to fade as Roman's enemies changed. I mean with that huge shield, Romans were slower than a turtle compare to their enemies such as Huns and Persians.
Lol, Persians?
Also, in the late roman times, the whole army concept changed. Legions no longer garrisoned the line itself (such as the Rhine Limes), but were held back in centres away from the frontline, ready to reinforce positions in times of need. As such, it might have been more useful to shift to shift to lighter equipment?
