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I think that it is at least partially due them costing more to make, so they do not sell as well to players.
Of course we could argue 'chicken and egg' on that observation, but the process to make flats IS more complicated and takes longer, so I would suggest it to be somewhat true.
The economies of scale [of making rounds vs flats] may not as much be a factor, because as I understand it, it is harder if not impossible to get a machine to make the final wrap on a flatwound string and to polish it, without at least some human interaction, according to multiple string makers which brag about their flats being hand-polished.
Then, makers of basses exacerbate matters by choosing the lowest priced string which they can for almost all new basses [so roundwounds], and rock players have grown to like rounds anyway over the years.
So everything which I have mentioned contributes to your initial thought.
BTW, add to this that there are fewer players of short and medium scale basses [let alone extra-short scales like the Squier Mini-P], and it yields a perfect storm' as to why players of those basses can find fewer choices of bass strings in general.
It all comes back to money, at some level.
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