Image that there are several different ways to meet these energy requirements, and mitochondria just happened to occur first. Mitochondrial life was first and so it had a chance to evolve and optimized itself. Any new systems that arose through random mutation would not have had a chance to evolve into a high-efficiency configuration and would not be able to compete.
Other potential systems may exist and simply never got the foothold necessary to evolve into an efficient competitor. I would guess that it's quite likely we're at a local maximum, not the single and only possible way to organize cellular life.
The majority of Earth's history was spent in a state that contained simple single-celled organisms but no eukaryotes. Once eukaryotes showed up, everything changed rather abruptly. This is strong evidence against the idea that other alternatives would have arisen had we not been here to compete.
Why? There's a whole kingdom of really bizzare life, called Archae.
Also, the notion that prokarya are simple and uncomplex is totally bunk. There's a whole genus of prokarya - actinomycetes - that have such a complex lifecycle, and stage through single-celled spores, and multicellular superstructured colonies, that for many many years, scientists thought they were fungi.
"Simple" is quantifiable as how much information-theoretic complexity a population's selection pressure can sustain against mutational entropy to allow for relatively permanent (on non-evolutionary time scales) adaptations. It's not a measure of how much an organism makes us say "Gee whiz!" This is what I was referring to.
Hydrogenosomes. Hydrogen fixation. Nitrogenosomes. (as in annamoxa). These energy requirements exist. There are really wonky bacteria out there that have developed all sorts of interesting additions to themselves.
The same argument explains why there are not multiple heritages for prokaryotic life as well, even though we would assume that similar conditions that allowed for life to arise in the first place should be giving rise to simple pre-cellular proto-life-forms even today. But any such organism would be massively out-competed by existing life forms, and thus not get a chance to evolve sufficiently to be competitive.
Other potential systems may exist and simply never got the foothold necessary to evolve into an efficient competitor. I would guess that it's quite likely we're at a local maximum, not the single and only possible way to organize cellular life.