Tim Hardwick, MacRumors:

In a bid to harmonize standards, the EC negotiated a 2009 Memorandum of Understanding that was signed by 14 tech companies including Apple, Samsung, Nokia, and other prominent smartphone manufacturers.

And:

However, the wording of the MoU offered Apple a loophole: For those phones that did not have a USB micro-B interface, an adapter was allowed under the agreed terms. And that’s exactly what Apple did. In 2012, Apple introduced the ‌iPhone‌ 5 with a new Lightning proprietary connector to replace its 30-pin connector, and additionally offered a separate Lightning to micro USB adapter to comply with the 2009 EU agreement.

And:

By 2016, the Commission acknowledged that micro-USB had become dated and that USB-C had become the de facto standard across most devices. The Commission was advised by MoU facilitators that all manufacturers were ready to sign a new agreement in line with different approaches but keeping the solution of using solely USB-C connectors – except Apple.

At least part of Apple’s argument against an on-device USB-C port is that adding USB-C would make the phone thicker. They also argue that a dictated standard would hurt innovation.

Personally, the variety of ports out there, and the differing cables and dongles I’ve got to keep around, especially when I travel, is painful. I just came back from a trip where I rented a car equipped with a USB-A port. I realized that all the bricks I’d brought with me were USB-C bricks, leaving me with only Lightning to USB-C cables. Annoying.

The linked article is a well-written take on the current situation with the EU and Apple’s position in opposition.