Why are H+ ions pumped out instead of in during cellular respiration
Mitochondria pumps out the H+ perhaps just because there is no disadvantage of it. Once they've evolved such a machinery with complex network; they weren't threatened to evolve any opposite-system.

Any opposite system too; would not plausibly disadvantageous; and really happens in case of chloroplast; another sort of cell-organelle. 






1: image from Plant Biochemistry, by Hans Walter Heldt, 3rd Edition.  

2:  picture including all chloroplast membranes. 



But if any organelle used cytosol as the H+ pool; probably that would made cytosol acidic as wel disrupt metabolic processes on cytosol. (usually the cytosol have very mildly alkaline pH.)


  Reference: Plant Biochemistry by Hans Walter Heldt, 3rd Edition, Academic Press.