Because DSLRs are dead. If you look at the new mirrorless cameras, they're either doing USB-C charging, or take batteries charged from a device that uses USB-C.
The cameras are just a random example.
As soon as you look at the EU’s decision (which is one I like the effects of) you can see that this was just a very arbitrary ruling.
> The cameras are just a random example.
And you got your answer for it.
HDMI is a somewhat-open-ish, widespread standard and for what it's worth Apple MacBook models haven't carried HDMI ports for ages now.
> Why are dSLR cameras allowed to continue to use micro USB and SD cards when they could just use USB-C so we don't have to buy dongles and adapters?
Again, the market seems to converge towards USB-C already (almost all the models on [1] and other buying guides have USB-C), and for memory cards towards SD cards and CFexpress (on the pro segment). Both of these are somewhat-open-ish again.
The EU usually only steps in when the free market is either stuck somewhere along the road and needs a push towards a specific standard (=> USB-C), profit incentives need to be overruled (=> RoHS / leaded solder) or the free market suffers from oligopolies that need to be broken up for actual competition or the creation of an actual single market (=> EU-wide mobile roaming).
> The argument that Apple only provides one physical port (they also provide Magsafe and allow other wireless charging options) isn't sufficient with respect to your discussion point about market-wide standards.
Apple can provide a MagSafe port on MacBooks or a Lightning port on iDevices if they want. All Apple has to do is to support the mandated USB-C port as well!
[1] https://www.digitalcameraworld.com/buying-guides/the-best-mi...
These ones do as they were re-implemented per customer demand: https://www.apple.com/macbook-pro-14-and-16/specs/
When will the EU ban HDMI connections to monitors or force Apple to remove them from MacBooks? Why are dSLR cameras allowed to continue to use micro USB and SD cards when they could just use USB-C so we don't have to buy dongles and adapters?
The argument that Apple only provides one physical port (they also provide Magsafe and allow other wireless charging options) isn't sufficient with respect to your discussion point about market-wide standards. This really looks to me like the EU is just saying "we don't like lightning chargers and since nobody is going to defend it except Apple we'll ban it" while they also let plenty of other "standards" run amok in the market.
FWIW I much prefer USB-C and I want my next iPhone to include it (along with any other devices), but I really struggle with the logic of the decision to force it here.