They were the first pro sports team and have the most home grown fans.
Cardinals are second but were a transplant from another city and have been bad for most of the history. They’ve only gotten better in the past 15-20 years.
In addition, Arizona is a huge melting pot. Go to any Diamondbacks game and there are likely as many fans for the road team as the home team.
Suns fans have always been around, but recently in quiet misery. We finally have something recent to cheer about, so we are.