The anime is produced exclusively to sell toys. Rowlet is the most popular Gen VII pokémon, its final evolution, not even in the discussion. They wanted to have an excuse to keep the kawaii, comedy relief owl around as they were able to add merch lines they hadn't even conceptualised at the start of the Generation, with how the anime presented Ash's Rowlet. In short, because borb makes more money for the franchise than its final evolution ever could.
