Because those two barely breathing dinosaurs bring in ratings every episode better than all but Supergirl's premier may have something to do with it.
Honestly renewing Supergirl is a bad idea it will never reach 4 seasons so will never get syndication. Hitting 1.3 in a first season is terrible so unless they retool the show just put it out of it's misery now rather than watch it limp to fractional in it's second season.