Brady just went to a team that had the worst winning percentage of all four major sports, hadn't made the postseason in thirteen years, and finished 7-9 last year, and 5-11 in each of the two preceeding seasons. Bruce Arians was widely panned almost all season as being the reason the team will always be a pretender.
How about Mahomes? Hmmm...let's see. He took over a Chiefs team in 2018 with a head coach sporting one of the best winning percentages and best reputations of the modern era, though his quarterbacks were Donovan McNabb and Alex Smith. Reid took over a 2-14 Chiefs disaster in 2012 and had the following records under Alex Smith: 11-5, 9-7, 11-5, 12-4, 10-6. Smith, an average QB with big limitations by all accounts, also scored 40+ points on the Patriots in 2014 and 2017.
But yes, it was all about Bruce Arians and the Bucs being known for greatness and giving a quarterback a huge advantage, wheras Mahomes is the reason that the incompetent buffoon Andy Reid even has a job.