20+ years of peace and prosperity by a man of great humility may not excite the ancient gossipers, but the ‘mildness of Antoninus’ was fondly remembered by those who lived under his rule.
Also, he hit the holy trinity of fiscal responsibility. Reduced taxes, increased spending, and still reduced the deficit. That’s like hitting three 7s on a slot machine.
As a side note, Antoninus Pius was deeply ‘traditionalist/conservative’ in a Roman sense - meaning he preserved out-of-fashion republican norms from the days of the Roman Republic, did not push new cults, gods, or rites (unlike some prior Emperors), and relied on the Senate as a partner in rulership rather than cultivating an extensive imperial court.