Why did the Roman Empire abandon plate armor (forgot the term for it) and instead slowly adopted scale and chainmail over time?

The Romans never had Plate. What you're probably thinking of is Lorica Segmentata- which is basically sort of modified reinforced Lamellar (not exactly TBH; completely different in style).

Lamellar might offer marginally superior protection from penetration than Mail, but the latter has advantages in maintenance and late Classical weaves could “seal” together from gravity, providing often better protection with extra flexibility.

In addition, there's zero reason to believe that LS was, at any point, more prevalent than Mail over the entire army. Ergo the “shift” from LS to mail never happened.