In a word, FAIRNESS. Pellets are easy, they all shoot about the same speed with a BC of about the same value. The shooter is the deciding factor...
Longer discussion below:
I also think its because slugs while gaining popularity, still aren't mainstream You may argue that, but they really only popped to the top of everyone's radar within the past year or so. Sure, some guys will pop off that they've been doing it for 23 years now with their keyboard guns that shoot bottle caps or flying bumble bees at 200 meters, but generally its only been recently that you could buy slugs for a reasonable price (thanks Nick, NSA Specialty Ammo). As many slugs as it takes to tune a gun for the best slug at the best speed, paying $20 for a hundred was a steep entry price. Also, I think some of the ranges really aren't set up to go further than 100 yards, which is where slugs would shine. The small bore range at Rio Salado (EBR) isn't.
There's the challenge of it. Think about it. No one has shot better than 225 in 100 yard competition at the finals of a major tournament (EBR, RMAC, PAC). Slug scores would be better, but lets face it, no one is going to shoot 250. Those that don't shoot slugs might think they go straight and aren't affected by the wind. Wrong! They are affected by the wind, just not as much. An FPE limit would be necessary for the game to remain equitable. Whereas with pellets, aside from that sprinkling of luck with the wind, the best shooter has much more influence on his score than does his gun, but the opposite could be true with slugs. Pay for the best gun/slug combo and you can win, simple, right?
Of course then there's fairness in competition. Do we limit the slugs to a certain FPE?. How many classes do we need, or what is fair? Allowing slugs at 100 yards would end up in a "space race" with equipment costs and development time skyrocketing. Whereas now you can buy a pellet rifle, decent scope, practice and be competitive, that wouldn't be the case with slugs, at least not yet. So unlike pellets, where "mo' power" really isn't a benefit, and the playing field is fairly level, what happens when Power Hungry Pete shows up with a .35 cal gun that shoots 110 grain slugs at 1070 FPS and have a BC of 0.28? Would that be fair to the average Joe that wants to compete with his .25 cal gun shooting 26 grain slugs with a BC of 0.085? There are so many more variables with slugs, it'll take time to figure it all out.
I think that the major tournaments actually will be incorporating slugs. Maybe not yet for the main event, but look at RMAC, they have a small bore slug challenge, 100 to 225 yards, shooting steel. I'm sure EBR 2020 will have a new event where small bore slugs are allowed, or required. I don't think the big bore event will ever be very popular, its more of a niche side show event. But small bore slugs will continue to increase in popularity and the tournaments will give the competitors what they want...
