As already mentioned above, the cameras other internal hardware limit the data rate we can put to the HDMI out (bandwidth and firmware limitations can be dictated by other hardware in the camera, not just the output port). We also have the limitation in those cameras that we can't do ProRes 444 and have to stick to Prores 422HQ as the highest quality ProRes option for similar reasons; limitations in hardware/bandwidth/etc.
The cost of adding hardware to support this would put the price into a completely different category (not to mention generate more heat and use more power) and those things are always weighed up in the decision making - while 4K output and ProRes 444 will make a difference to some people - will it make a difference to the majority of users the camera is aimed at? If not, we can put the camera at a much more affordable price point. Sometimes tough decisions like that are made to reach a target price point, especially if we have other products in the line that may be better suited anyway. And then sometimes the cost of hardware (and its power/heat effeciency) comes down over time and we can introduce new models that add features that were previously not cost effective. But as you say, we don't like to limit our products and try cram everything we can in there. But we don't always get everything we want in there either because you have to be practical at the end of the day.
Its the usual story for any hardware manufacturer of any product.