Here's my theory (and it's only that): Brick and mortar stores mostly cater to beginners and (uninformed) impulse buyers, particularly to parents buying essentially toy starter instruments for younger children. Also, ukes are relatively slow-moving items, so stores don't like to tie up much money stocking ukes. Sopranos and concerts are cheaper than tenors if only because less material is used to make them, so a store can stock three sopranos for the same outlay as for two tenors; and in the popular mind, ukes "should be" small, while beginners who are unsure of the right size for them will opt for either the over-touted "standard" (soprano) or the middle compromise of the concert. It's only after this first exposure that people have a better sense of what size they may actually prefer, at which time they'll probably also be looking to upgrade, and will have found the stock at local stores to be too limited, so they're more likely to scout online, where they can find all the sopranos, concerts and tenors they want, even if they can't test-drive them.
In truth, you find a wider range of options in tenors than in the other sizes, even though there are some companies (like Kiwaya, last I checked) that make only sopranos and concerts. The fewest options exist for baritones, treated as the bastard siblings of the uke family.