I could tell you too many reasons people have speculated: “too picky,” “too accomplished,” “too educated,” “too busy to date,” “too heavy,” “to plain,” “too simple,” “no major yichus,” “too fum,” “not yeshivish enough,” “too closed minded,” “younger siblings skipped ahead,” “wants a Moshe Rabeinu,” “went to the ‘wrong’ schools,” “davens in the ‘wrong’ shul,” “too quiet,” “too talkative,” etc.
The real reason is because Hashem has His master plan, time, and place for everything.