Most Unity games are mobile apps. Some suits probably saw the numbers on installs and saw dollar signs, but never thought about if it was a viable pricing model for most of their actual customers.
I guess they don't really value customers who can only generate $0.01-1.0 or less per install. Not saying it's right but it's not like this group of developers is really paying them that much anyway (and noncommercial games/apps don't seem to be affected at al).
I'm sure one reason Unity did this is that are pretty certain that many of those developers won't be willing to share their revenue. In countries like China where even relatively large studios would just pay for 1-5 licenses for 100 employees forcing them to give you a share of their revenue seems hardly possible.
If they are already clients and paying for Plus/Pro/etc. subscriptions that shouldn't be too hard (much, much easier anyway).