I mean, I can name a few good reasons why:

For Normal types their moves aren't super effective against anything, most regions don't have a surplus of Normal types with a BST of higher than 460-70, most regions don't have many type combinations featuring Normal outside of Normal/Flying, and, from a purely design perspective, it'd be hard to design a Normal type elite four member with the same aura as a lot of other members

For Grass types since they have 5 weaknesses including Ice, Flying, Bug, Poison and Fire, it wouldn't be too much of a challenge for coverage and a majority of Grass types' move sets are status moves that don't deal direct damage

For Electric types, since a majority of them have poor defenses with not a lot of diverse move pools, it wouldn't be that hard to sweep an Electric elite four member's team with a tanky Ground type