CW is also cancelling/ending some other shows, like "Supernatural (2005-)" and "The 100 (2014-)". One of the main reasons "Supergirl (2015-)" is cancelled is its abysmal ratings. Since 2015 the average viewership has dropped more than 83% (US: 89%) from (S1) 9.81 million (US: 7.86 million) to (S5) 1.58 million (US: 0.84 million). If you only consider the numbers since it moved from CBS to CW, they are more favourable: (S2) 3.12 million (US: 2.36 million) to (S5) 1.58 million (US: 0.84 million); that's 'only' a a drop of 49% (US: 64%).

"Batwoman (2019-)" numbers for season 1 are quite low too: US average viewers 0.93 million, but it will get a second season.

Side notes:

Kara Zor-El and Mon-El are having a baby. Is Kal-El going to be there?