Many of these SoC have HDMI directly from the IC without needing additional hardware converters. Something like DP you need a converter IC which adds expense and board space/complexity. Something like MIPI -> DP, I think TI makes some chipsets that do this.

The size is probably to save space on the board - looks like they try to keep to relatively the same size and mounting holes as previous boards.

I have an entire box of these mini-normal converter cables because of boards like this. OK when messing around but I can see the frustrations if you're not used to it.