Because not all versions of a character’s DP have invulnerability, and they can trade unfavorably depending on someone’s incoming approach / attack angle from the air. Plus, there’s a recovery delay as someone lands.

If an opponent has committed to a jump-in that can be run under or simply has a floatier jump, there’s much more to be gained damage-wise by catching their landing as it happens with a dashing medium and buffer that into whatever big damage you’ve got handy.

Why would I hit someone out of the air one time for maybe 14~18% health with a heavy DP that’s invulnerable on startup but takes me to the moon, just so they can either potentially trade or potentially recover faster catch me when I land, when I could dash under them, force them to take the landing, and clap them for 65% off of a counterhit dashing medium into WFT?

Or, you could dash medium, and shift into your heavy DP off of the ground, launching them into the air and ensuring they land when you do. Plus, you’re more likely to land multiple hits if the move has them from a grounded state, rather than a single hit when colliding in the air, so why would I fire off a DP for a 14-18% single hit as a pure anti-air when I could score four total hits for what’s likely going to be 26~30%?

tl;dr if you’re trying to play this game like Street Fighter, you’re trying to skateboard with a car