I love somebody's explanation on SWF that they posted and I'm going to post it here because it makes so much sense.
There's no such thing as priority. What most people think of as "priority" is just a combination of timing and disjointedness. Fox's drill kick hits almost constantly, and has quite a large set of hitboxes (test in 1/4 speed vs Kirby, whose get-hitboxes match his looks closest, IMO).
Sex kicks are considered "high priority" because the timing is absolute. They hit all the time in a certain window. To hit someone through a constant hitbox requires that your hitbox be more disjointed AND you time it correctly to take full advantage of the additional disjointedness. Zelda's f-smash is so "high priority" because it has several hits, and is notably larger than her arms.
Peach's dsmash IS disjointed. Her dress can't be hit, but it can hit opponents. She doesn't put herself in ANY additional danger (distancewise) by performing this attack. That's how I define disjointed. Samus' dsmash involves her sticking her leg out. It's not invincible; it can be hit. Nearly all attacks are disjointed to a degree, but some just plain aren't attached to a vulnerable body part.
Credit to SuperDoodleMan.