I will structure my answer into three parts.
In the first one, I will be vaguely railing against the arbitrary restriction on the cost and other economic factors )
The issue here is not only in the straight up monetary cost. The biggest resource is the work time. Late plate armor needs a lot of effort of a fairly specialized professional armorer to produce. It's not a task that can be scaled by throwing more people at it, unlike the production of maille, for example. It also needs a lot of personalized fitting. So, it's not an item that can be taken 'off the shelf' or from the municipal armory - it needs to be ordered in advance by a person who plans to do nothing but fighting in the near time, or even for most of his life.
Another factor to consider - in the medieval army an armored knight needs a fairly large retinue. However your army is composed, someone needs to care for the horses, forage, build the camp, dig fortifications and latrines, help the knights to put the armour on, and so on. And unless your warfare is heavily ritualized form of mass joust, where non-knights are out of bonds and can't be attacked, it makes sense to arm and armour at least some of those people. So, every infantryman and squire would also need plate armour by your logic. Time being the most precious resource, you are not able to equip every one of those with fitted white plate, so by necessity munitions grade armour enters the equation - and it is 'worse' then knightly armour, being either heavier or less protective or both.
Moving to my next two points, I need to say that I actually do not quite understand your question. Seeing your replies above, I can't say which of the two are your asking:
1) why would any individual person choose 'inferior' armour if he has an access to full plate?
2) why would a nation in a world where plate armour is available, armour it's heavy cavalry and heavy infantry in inferior armour?
So I will try answering them in turn.
As for the first variant, the answer is party covered above. Any army has many more roles then heavy cavalry and heavy infantry. Actual fighting takes a minuscule proportion of the time any warrior or soldier spends in the field. Most of the time is spent marching, riding, putting up camp, sleeping, eating, standing sentry and dicing ) However comfortable full plate may be, you can't spend 24 hours a day in it. Some soldiers can opt for armour that is easier to put on and take off (one of the advantages of the brigandine, actually, is not the cost, but the ease of putting it on by yourself), or easier to do daily tasks in. You can dig latrines in full plate, but it's absolutely not optimal way to spend your time. 'Inferior' types of armour can have other logistical advantages - ease of field repair, for example.
Also, the soldiers that are not expected to enter melee would not wear full armour even if they had time to prepare. You can even see that on the the medieval miniatures the archers are drawn wearing full plate, but with open faced helmet and without gauntlets. So, archers, pikemen, siege weapon and artillery crews would not wear full enclosed plate.
The main idea in this part is - soldiers often sacrifice protection for comfort. And throughout the history, the average infantry armour is some sort of torso armour and open helmet. So unless the soldier absolutely needs full armour, he won't use it. It goes double for 'adventurers'. Unlike soldiers, they won't have an advantage of having the baggage train where they can stash their armor. If you have a group of adventures, one of them could be clad in full armour, while the rest serves as his retinue, helps him carry it, and protects him, while helping him to put it on.
Whatever the gripes people may have with good old Tolkien, the Fellowship is one of the best description of a group of adventurers in terms of their traveling kit - they have only two shirts of maille among them, one is worn by superhumanly strong dwarf, another weighs almost nothing, a single sidearm per person, while the main bulk of their belongings is spare clothes, ropes, water, food and occasional firewood.
Compared to that, the more weapons and armor your adventurers have, the more unarmored helpers they need - servants, native guides, packhorses etc.
The answer to the second variant of the question is more complicated. It's also hard to answer, why a nation would not use full plate armour without resorting to the economic explanations.
The easiest case is isolation. The country is pretty isolated, it had it's own martial culture, own tradition of arms and armor and had only recently come into the contact with the 'mainstream' cultures of your world that use plate armour (think 16 century Japan).
Some nations can be in the perifery of your world, or these are the peripherical regions of bigger nations. They have a constant low-level conflict with nomadic tribes, so mobility is a higher priority over protection (think the whole Eastern Europe on the border with Great Steppe for the most of the history).
Some regions can be very distinct geographically, so it's hard to use heavy cavalry there - broken terrain, bogs, mountains, etc. (think Scottish Highlands).
So, some specific local factors can prioritize light infantry or light cavalry as the backbone of the military in the region.
Different economic factors can also be in play here. The particular region can be poor in local resources, or not urbanized enough to produce plate armour locally. While it is possible to import, it adds additional complications, so that poorer local armour still becomes a more 'optimal' choice in a lot of situations. If you need it as a plot point, there can be even a trade embargo on a particular nation, where everybody agrees not to sell their better quality arms to them.