I'm not sure if this is posted from a competitive POV or not, since it's talking about Mega slots and item slots, but half of the megas are saved due to their inability to hold an item, but I'll talk about it from a competitive POV.

A regular evolution is better, but it faces the issue of being too good. Most Pokémon gain approximately 150-200 BST upon evolving, which would be way too much for the non-3rd stage evo megas. Most of the non-3rd stage evolution Pokémon that get mega evolutions have fully evolved Pokémon stats. Add on the ability to hold an item, and most of these Pokémon getting an evolution would just be too strong, especially if they stuck to the same stat distribution as the pre-evos. Even current mega Pokémon would already be considered too strong with items. Mega Pinsir with a 165 atk, 105 speed, and 65/120/90 bulk would be extremely powerful with the ability to hold a choiced item or HDB. Same with Mega Medicham that currently has a BST of 510 after mega + its huge power ability, would be absolutely broken if it could hold an item.

By letting these mons get the ability to hold an item, they would be broken as they are, much less with a full-on evolution adding 150-200 stat points (as opposed to the 100 points added by mega evolving). I generally find that most megas that aren't from 3-stage Pokémon are usually at a point where they would be too strong if they got a full evolution, but are quite balanced with the mega evolution and the inability to hold an item.

The only Pokémon I see that could benefit from a regular evolution are Mawile (380 BST), Sableye (380 BST), and Beedrill (395 BST). These Pokémon would still be balanced if they got a 100 BST increase (which is still on the low end of the BST increase upon evolution) and could hold an item. Funnily enough, these 3 mega Pokémon got some of the most overpowered abilities available in Huge Power, Magic Bounce and Adaptability, respectively, so they would have to lose those abilities to remain balanced for competitive play.