Because it's better for the consumer. If you only have 1 connector type, then you don't need a different charger/cable for each phone. Nor do you need to worry if you move from, for example, the Apple ecosystem to something else.

That saves the consumer money and is arguably better for the environment because fewer chargers/cables are required (plus additional transport impacts, packaging etc.) If this expands into other devices - tablets, cameras etc. then you see how this really ramps up.

It's not the first time that the EU has taken a pro-consumer stance on technology. Those of a certain age may remember the EU having issues with Windows coming bundled with Internet Explorer, just as one example.