Everything was cloakable for a while, until December 2012. Upon reflection we realized that the primary purpose of cloaking was to cloak structures and that there was no class of structure which was fun to cloak. Consider some types of structure.
Energy - Past a point you may as well cloak all singus and fusions. Unless these structures are spotted during construction they are effectively impossible to scout so deny your opponents both targets and the ability to compare economies.
Turrets - Structures cannot move. Once a structure decloaks to fire you know exactly where it is. When it recloaks it is hiding nothing from the player, it is just making the players units act stupidly (since they will not automatically fire at the structure which is known to be there).
Construction - Caretakers and factories are going to always be running. Technically you can halt them upon detecting an enemy scout but it seems hardly worth the micromanagement.
Nukes - Undetectable unless spotted during construction. Nukes can be stockpiled secretly and the launcher is basically unscoutable even when it fires. This is not OP though because a few cloaked antinukes make it just about impossible to get a nuke through.
In short, cloakable structures invariably reduced player interaction (through a massive nerf to scouting) and sometimes just causes unit AI issues.