The main qualification criteria for professionals are one of the folllowing — this off top of my head, so I can be missing a few more obscure ones;
Past winner
Top 50 in world ranking
Qualified for 2020 fedex cup final 30.
Winner of any tour event last year
Top 16 (or 12?) in 2020 masters
Top 5 in any 2020 major (might be wrong on this one).
but basically Fowler made it every year because he has always been top 50 in the world. He really struggled the past 15 months like Speith and is no longer in those. Except Spieth turned it around for the past 3 months with numerous stronger finishes — he was getting in good from before his ultimate win. But Speith slump was so bad that he almost fell out of the world top 100.