On 135 service that limit is 10,000 ft, not 12,500 or 15k. I'm assuming 121 is the same.
As Arnold noted, the short answer is you don't fly that high, and they were planes primarily used for short hauls where you needed the size to have the capacity (and thus the power to move the size) but you didn't need pressurization. Another similar aircraft was the Shorts Sherpa. Pressurization adds weight and cost, and you're not going to fly that high on those short flights.
There are a lot of remote airports where this makes more sense than flying a regional jet, at least to me. When I lived in Pennsylvania, the one flight they had (going to PHL) was in -8s. Heading to PHL you flew at 7,000 ft. Coming home you'd go at 12,000, but really no reason why 10,000 wouldn't have worked fine as a limit if required. Pressurization really wasn't needed.
The Caravan and the Kodiak are pretty much what remains of that general type of market, although the Kodiak is really more of a rugged aircraft for use in unimproved areas.
The market has just moved away from turboprops almost entirely in favor of jets, for reasons I don't fully comprehend.