Why did Roman armies adopt the Spatha & abandon the Gladius?

The Roman military of he Republican and Pricipate periods seemed to have a winning combination of gladius, and scutum (the rectangular Roman shield). And yet, starting at some point in the late secondr early third century AD, both of these began to be phased out, and the Romans would adopt a new sword--the spatha. Why did they do this? This video atttempts some sort of an answer.

Видео Why did Roman armies adopt the Spatha & abandon the Gladius? канала The Historian's Craft