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possibly because fan artists don't have the pressure that the official ones do
since the games are on a strict time limit and have to release with new designs every ~3 years, you have a set period of time to work on designs for and by the time you're done you're immediately forced to continue on a different set
whereas fan artists can easily spend years on one region, refining and revising, diluting the series' (old) design hallmarks into a fine set of guidelines to help them create designs that fit in with the aesthetic
it also helps that fakemon have been a popular and prominent thing for a LONG time now- it's been long enough that there are plenty of examples to draw from and stereotypes/beginner traps (in designing fakemon) to avoid, as well as established community pockets that are constantly formulating how to make the designs "better" and more in-line with the series and such
these fans have also grown up with the series for their entire lives and have a much, MUCH larger pool of official pokemon to draw from for a standard as opposed to the pokegod creators of yore who only had the first one or two batches to go off of
tl;dr: fan-artists probably have more time to make sure their designs look good and fit with the series, as well as much more reference material to take inspiration from or refer to when necessary
as opposed to the official artists who are probably crunched as fuck and being worked to the bone just for a first draft to get shat out the door for merchandising purposes
