I think in general many absences in representation can be placed on marriage being a rather late addition, and the distribution of potential spouses being arbitrary.
From what I can tell, Bethesda was originally going to allow you marry most anyone, but quickly ran headlong into problems of station, AI schedule, and quest involvement, so they rather arbitrarily designated NPCs are eligible or not (which explains why there are no Bosmer spouses, given that they use generic voice types).
As for Khajiit, there are no female Khajiit with favors and hence there would have been no female Khajiit potential spouses (likely, while making the marriage quest, the designer forgot Kharjo and J'zargo and was under the impression that there were no Khajiit at all with favors and therefore no potential Khajiit spouses; if this seems unbelievable, remember that Bethesda's team has different subsections working on different things at the same time -- the fellows who worked on the College things was probably not the same person who created the Khajiit caravans was probably not the same person who created the marriage-controling quests).