If you mean "why an SAO cz75/226/etc vs a 1911"?
1. Retain most/all the "easy to shoot well" characteristics of a 1911. You get a consistently short light trigger press, plus the safety of a manual safety.
2. More reliable and durable than a 1911, especially one in close to the same price range.
3. Double the capacity of a 1911.
4. Much less expensive than a comparably reliable 2011.
5. Much cheaper mags/parts vs 2011, aftermarket parts more likely to just drop in.
6. High volume Competition shooting where the above benefits become very noticable, especially if you dont have the coin for a pair of 2011s plus mags.
If you mean "why an SAO P226 vs a TDA P226" (or insert other TDA gun)
1. Easier to learn to shoot well since no DA pull. This is arguable, but certainly easier to shoot with minimal proficiency because no DA pull.
2. Some might feel safer w/ a manual safety vs a decocked gun.
I know this wasn't your question, but if "why SAO P226/cz75/etc vs SAO striker with safety such as M17 or M&P"
1. Cleaner and probably shorter/lighter trigger press.
2. Subjectively enjoying the feel of a metal frame or a hammer over a striker.
3. Ability to ride hammer when reholstering.
Btw not saying the above reasons are necessarily true, just that they are possible answers to someone's Why. I used to run TDA based platforms cocked/locked for some of the above reasons, now all my serious guns are TDA W/ decockers or some kind of DAO.