There are tradeoffs to both.
The upright stance has a lot of benefits in terms of target to target transitions on static targets from a static position. That's the majority of IPSC and IDPA shooting. It's not a particularly common situation outside of those games; we don't normally see people standing out in the open, feet planted, engaging three or more close range immediate threats.
Once you start to move, being upright costs you. As an example, watch
this video of Bob Vogel starting at around 4:45. In particular, notice the difference between his stance when engaging the first set of targets (static) and the second (on the move).
Standing upright also has a cost in terms of recoil control. That isn't as big a deal to world-class competitors whose guns are tuned to recoil the way they want, who have good upper body strength and great grip strength, etc. As I tell students all the time, if you want to see what
ideal technique is, look at top
female competitors. They are normally lighter, have smaller hands, and have less upper body strength then their male counterparts. Here are photos of two of the best known women in the action shooting sports. Notice how radically different their Open (red dot, compensated gun) stance is to their Production/IDPA "gun type" stance:
Julie Golob, standard gun
Julie Golob, compensated gun
Jessie Harrison, Limited gun
Jessie Harrison, compensated rimfire
If you look back, you'll see the upright stance thing really got rolling first in Open division, even among men.
If you're not concerned with large arrays of static targets, if you haven't perfected recoil management, or if you lack the physicality of the the guys who are winning the top spots in major action pistol sports, the upright stances makes less sense. Furthermore, if you're not going to practice your static position shooting
and your shooting on the move to the point where you'll naturally, preconsciously switch from upright to crouched when you need to start shooting on the move (i.e., 99% of the shooters on Earth) then spending all of your time shooting upright tends to interfere with best practice for shooting on the move.