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I'm guessing for the same reason why Tesla moved to the GB connector for Superchargers in China: politics. Since CCS is becoming the national/European standard, Tesla is converting to the standard to show support. (In China, this decision may not have been voluntary.)
It might also be more future-proof in order to support faster supercharging, since the CCS standard is designed for high power outputs, whereas the modified type2 has not demonstrated this capability.
And stepping back from my Tesla owner bias, I have to admit that having all plugs being the same in a country does have immense benefits in the long run. Take a look at any electrical appliance, for example. In the long run, the number of superchargers using a proprietary standard can never match the number and scale of multiple operators a full market. This is already the case in China, where superchargers are far outnumbered, and is true in other domains as well (Android vs iOS for example).
The downside to this is that existing S/X owners will now be forever inconvenienced at superchargers.
Just saw this post regarding CCS in Australia: CCS Charging options for Model 3
This further supports my speculation on the political side of this switch to CCS. Tesla wants to be seen as supporting public standards.