I don't understand why the main series can't be this good. This series is great.
If you mean the art and animation, a movie or short/special will ALWAYS be better animated than a a TV series and ESPECIALLY a long-running one.
Time and money.
Short series can have artsy animation, the ones that have maybe 13 or 24 episodes. The TV anime has been going on for two decades... it's far harder to spread the budget over the course of that and they're often still working on the episode until the day it airs!!
Specials like The Strongest Mega Evolution (one episode each half year, four Acts/episodes in total) and now Twilight Wings (one episode each month, as I recall reading?) have more time and can better use the budget due to being shorter.
Please don't devalue all the hard work that goes into making the TV series, it's just that this is what comes of a long-running TV series. That's also why it often dropped into being formulaic (it'd be hard on anyone to keep coming up with unique plots for all of these creatures on a regular basis, many have been working since the start!).
XY, Sun & Moon, and Pocket Monsters (2019) look so good because it's cheaper now to make good animation, but there is still a lot of effort because all of these key frames you're watching were drawn on paper first, by hand...
If you mean in terms of story (there isn't a LOT of one yet, but), it's likely due to the writers being less constrained in subject matter because these specials are usually aimed at older fans, whereas the TV series is classed as kodomo/children and, even if it blends in shounen and sometimes shoujo archetypes and plot elements (Contest arcs with the many different rival archetypes VS training, adventure, tournament arcs, etc.), they have to be able to air in roughly the same sort of slot that would air other family friendly/all ages entertainment, while winning over the tiniest kids who are the primary demographic too (like... the Hamtaro/Tama & Friends demographic) for merchandising purposes. ^^;