It's not so much the market base as it is the compatibility of a single generic barrel design to economically apply to many different brand's mounts. It's the price we pay for today's compatibility with half a century of older Pentax/M42+PK+PKA lenses. The same situation ended the era of Tamron's excellent Adaptall-2 SP lenses where one lens body and optical formula accommodated all brands via economical mount adapters. No one whines that older Canicon lenses are incompatible with their new bodies and that was an in-house evolution decision in order to move forward.
Pentax's longer flange registration distance (FRD?) also plays into this. Longer FRDs have more flexible design limits. How many modern FF or APS-C lenses have longer FRDs than Pentax's 45.46 mm? The longer FRD can accommodate a number of adapter widths between a body and lens base. That's why Canon guys like SMC primes.
When other brands adopted the new style electronic aperture control, and especially the new in-lens SR and AF functions, they had to accept a heavier, larger-diameter barrel and mount design which orphaned prior bodies. Third-party manufacturers can design a single body and optical formula to their new standards and add relatively little expense to customizing the various mounts IF they're compatible. If you wonder why Tamron/Sigma lenses sometimes seem to be a wee bit 'fatter' it may be to cover all potential contingencies with a single design.
(Note that electronic contacts greatly simplify the mount design issues compared to the older, analog/mechanical connections. Physical dimensions aside, one wonders where the Adaptall lens concept could be today given the flexibility of positioning l'il bitty wires, electronic contacts and adaptive computer chips into a unique mount adapter compared to the various fixed, mechanical monkey-motion mounts of the past. I wonder how many Pentax engineers have banged their heads on the wall wishing for an extra 3-5 mm of mount diameter over the older, pre-AF, PK-standard. Could that be one reason Pentax adopted in-body SR?)
So, yes, there's a market economics issue, but it's also about the difficulty of including the PK/A mount dimension standards within a single economical, third-party barrel design.
When you bemoan the lack of economical third-party lenses remember to thank Pentax for the other advantages of the venerable PK mount and its derivatives.
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