Not GF, so can't say for sure we know what their plans are. Generation 2 had weird design choices, scrapped content, and many odd 'broken pairs'; like Gloom getting a split evo but Weepinbell didn't (was considered and datamined from Spaceworld), and Vulpix not getting a pre-evo despite the Pokedex hinting at it (was also considered and was datamined from Spaceworld), and Farfetch'd's planned evo. However, the one thing that wasn't datamined from Spaceworld, was a Dunsparce evolution.
Granted, that doesn't cover all possible "left on the cutting room floor" ideas, but at the very least, through all the leaks, IIRC Dunsparce evolving wasn't a possibility they pursued.
If I had to hazard a guess, GF saw all the hype around fantastical evolutions, and decided to defy expectations.