Because those are old designs which haven't been updated in years? This may have a classic optical design (I hope so — the similarly-configured classic manual Pentax 50mm f/1.7 is one of the best 50s ever) but has modern coatings and modern aperture blade design.
Why all the lenses at 50mm f/1.8 for full frame from other branches have the prices around 100USD, while Pentax has 50mm f/1.8 for APS-C, with a smaller covering power, and a higher price?
50mm on Aps-c equals 75mm. A focal length which I not found much use for when I had the Nikkor AF 50mm 1.8D and used it with the D1H.
Wonder if this Pentax 50mm 1.8 might cover a future Pentax with FF. Probably not, so the question is how bad the corner falloff and vignetting is going to be like.
If I were going to complain about anything with the Pentax 50mm, it wouldn't be image quality. It would be the grinding sound of the AF motor. It's not a problem here at the studio shooting product shots. But I wouldn't want it at an intimate marriage ceremony. In those situations, switch to manual focus, which is quite nice on this lens.