Well, lets look at the sports. Who would be the rival in basketball- Utah State, whom they already play yearly. The WAC's best softball program has been Hawaii, whom they are losing in 2013. The baseball programs are usually subpar in the WAC, and women's soccer and volleyball in the WAC is usually laughable.
Now look at the WCC. In women's soccer the WCC had 3 schools ranked in the Top 15 last year. BYU gives them 4 of the top 15 from last year. In men's basketball both Gonzaga and St. Mary's were ranked at points, and San Francisco and Santa Clara aren't programs you'll beat with ease. Basically Gonzaga and St. Mary's are the equivalent of San Diego State and UNLV while San Francisco and Santa Clara are the equivalent of New Mexico. In women's volleyball the WCC had 2 programs ranked in the Top 25 last year. In baseball Gonzaga was ranked, though St. Mary's ended up winning the conference's title and stealing the NCAA spot. Who haven't I mentioned- San Diego, Pepperdine, Portland, and Loyola Marymount, right. Well, Loyola, Pepperdine, and San Diego were the three ranked soccer squads while Portland has already challenged BYU to matches in indoor track. The WCC will cost less than WAC schools (3 in Texas, 1 in New Mexico, and 1 in Louisiana). Plus as Tom Holmoe has stated, the WCC area features roughly 65% of all BYU recruits. Why wouldn't you want to feature your teams to potential recruits when you are in the area? The WCC is much better off for BYU in the long run.