Why are there convex and concave resistor arrays?
Concave arrays were once somewhat easier to make than convex.  They could also be made smaller for the same number and type of elements.  Some types of pick and place machines work better with concave arrays.  AFAIK, they have always had reliability issues in comparison with convex and discrete parts.  With modern prices almost negligible, most designers and companies have generally focused on convex parts since the cost difference is trivial.

There will always be some applications that need that extra miniaturization, or have highly aggressive cost goals for extremely high volume low end consumer goods, or haven't changed their design for 20 years.  There's also certain niches or designers that prefer convex for unusual or sometimes even irrational reasons.  Thus, the convex parts will likely be available for the foreseeable future.