What I don’t understand is this....why is women’s basketball the only women’s sport that the transfer rule applies to? I’d imagine from a revenue/popularity aspect, volleyball and softball have to be equal to basketball.
This wouldn't be true, especially for softball. NCAA publishes attendances by sport at their stats.NCAA.org site so you can see for yourself. Unfortunately they only publish by school without a consolidated average\total.
But running a few data points shows WBB is much better attended (all 2018-19 D1 data). For instance, WBB highest avg attendance is 10,406 (South Carolina); 17 of the high 20 programs avg > 5,000\game with little break after 17, and at the bottom only 76 of 351 schools avg below 500\game.
For WVolleyball the top attendance is 8,205 (Nebraska) but only 4 of 20 highest avg > 5,000 per game and there is a sig drop after those 4 such that the rest of the Top 20 avg out around 2,500 attendance. At the bottom 176 of 336 schools avg below 500 avg attendance.
For Softball it is just much lower. Highest Avg attendance is 2,544 (Alabama); only 2 programs avg > 2,500 per game and 253 of the 297 programs average less than 500 in attendance.
WBB and WVball play about the same number of games between 16 and 20 (with Volleyball looking like it is about 10% higher); Softball does play 2 to 2.5X as many home games.