It's simple economics.
2023 ave. attendance:
WNBA 6,615
NBA 18,322
2023 ave. ticket price:
WNBA cheapest team - expensive team $24-73
NBA cheapest team - expensive team $138-$611
Reg season games:
WNBA 36
NBA 82
So the NBA averages 2.28x as many games with 2.77x as many fans with a ticket that is 5.75 to 8.37x as expensive. So very coarsely that is $36.31 to $52.86 of NBA ticket revenue for every dollar of wNBA ticket revenue. And that isn't even counting the difference in ancillary revenue such as TV revenue, sponsorships, merch sales and F&B. Keep in mind that empty seats don't buy beer, hot dogs or popcorn.