Your limitation of unlimited resources takes out the most obvious answers. Is work going into the production of the armor not a factor either? Otherwise, this would be the most simple reason. Plate armor simply requires a lot of work hours from skilled craftsmen.
Without special circumstances there is no reason to settle for inferior protection, neither on a individual nor on a military level. If you can equip youself in a superior way without any disadvantages, why wouldn't you? (caveat here for militaries, sometimes the good old mass assault with arrow catching cannon fodder will still be superior, yet your resource catch negates this, unless manpower is also an unlimited resource or mass necromancy is a thing in your world).
Plate armor will also not limit agility or endurance significantly, as some people will certainly attempt to tell you. Even plate armor still allows you to do acrobatic tricks or scale a climbing wall (there are some cool videos of this in Youtube made by the Kingdom Come game developers). The endurence side of things can and should be fixed with training and drills. There might be people who are simply too weak to cope with plate armor, but they would fare poorly in combat against a plate user anyway, so there's no need to debate dead meat.
That all said, here are some circumstances where other armor might be chosen.
-Plate armor is ineffective:
This is what happened irl. Guns simply got too powerfull, and the performence of plate armor wasn't worth the money.
Steel plate doesn't like seawater, and swimming in it gets awkward real quick. Naval landings turn out horrible if your equipment drowns you. (This refers to permanent sea troops (aka sailors)). You might still use platy boys (this is a real word xD) for specific operations.
Strongly depends on the magic system. Yet if there is something some form of metal-bending (no pun intended) like in Avatar: The last air bender, plate armor becomes impractical and dangerous. On a similar note, many role play systems have metal interfere with spell casting, so there is a good reason why mages would wear cloth armor.
If you want to take a city with the help of infiltration forces, enemy guards will consider a bunch of people coming into the city in gambesons (which can be seen as normal winter cloaks), a lot less conspicuous than a bunch of platy boys. And if the mission will involve fighting, taking the most protection you can get away with is reasonable for individuals as well as for soldiers.
This applies more to individuals. If plate armor is readily available, local rulers might decide to make private ownership illegal to have a military advantage over the people. So any adventurer will be forced to use the next best tier of armor: mail over gambeson.
-Prestige and Sports:
In the Renaissance, rapier fencing was popular, as it was a test of style and skill. Platy boys bashing their heads in with halberds and half-sword techniques might be seen as clumsy and brutish. Going to battle (or rather duels) with no armor sends a statement and makes winning more prestigious. Maybe there is a tradition that dictates armorless fighting under certain circumstances.
Your infinite resource caveat limits this, but this is a serious limitation. Plate armor is maintenence heavy and heavy in itself. A Gambeson only needs sewing kit, but plate needs a forge and a skilled blacksmith to fix. Likewise plate might not be ideal for jungle warfare and especially an army needs significant infrastructure to supply plate to everyone.
-Tradition and Culture:
A weak one, as any culture that is technologically inferior will be conquered quickly, but "we've always done it this way" can be a strong argument.
I said no one would go for suboptimal armor, but in a medival setting plate armor isn't the strongest armor, plot armor is. Why would your hero, or any main character, bother with any type of armor if they can't get hurt anyway.
TLDR: Under your limitations there is no general good reason to settle for inferior armor. There are a number of exceptions, the only general non-quirky one being anti-armor magic.