I think because she didn't want people to see her underwear anymore. By the way i only watched Supergirl season 1. I know about that because i saw images of Supergirl
Supergirl is my 6th favorite DC superhero (behind Superman, Batman, Wonder Woman, Flash and Green Arrow), my 6th favorite female superhero (behind Elastigirl, Violet, Wonder Woman, Captain Marvel and Black Widow) and my 23rd favorite superhero of all time
According to Melissa it’s to symbolize Kara is “maturing”, but also because they film in Canada meaning ITS SO COLD. meaning a skirt VS pants I would choose pants
Well the whole Season 5 suit is a more "serious" or practical suit than the Season 1-4 suit.
Also, I think Kara's new suit is supposed to have incorporated elements of Red Daughter's suit, which has pants, since she merged with Red Daughter at the end of Season 4.
I said i only watched a few episodes of season 1, the 8th episode of season 3 and the 9th episode of season 4. I didn't watch all episodes and you just spoiled me the final episode of season 4