The WNBA only exists because of the ABL, a women's pro league that played during the regular basketball season. The NBA was having none of that - didn't want any pro basketball that wasn't played under their own auspices - so created the WNBA. At that time, part of the draw for players to choose the WNBA over the ABL was that it was a summer league - so they could play at home, then play overseas to make some real money - because the ABL, as a new creation, wasn't able to pay the players well, either. So, after just a few seasons, the ABL went bankrupt.
Would the ABL have continued to be viable if not for the WNBA? We'll never know how much of the ABL's financial woes were caused by the creation of the WNBA. But now the NBA has its women's league that it never really wanted, which I'm sure accounts for at least SOME of the reason why it struggles.