Yeah, that it took cranes to hoist up people in plate armor is a myth-yet another one that probably comes out of the Victorian era and fencing.

Full-plate armor was actually more flexible at the joints (except along the back and waist) than the human body and, torso aside, allowed for a greater range of movement than a human's own bones do.
While, yes, full-plate armor weighed an enormous amount, this weight was very distributed across the body. This weight was more an issue for stamina than speed or control. In fact, people skilled enough at moving in it were recorded to be able to run, climb, roll and with enough practice, even swim while in full-plate armor.
I've also linked that same 'armor aerobics' video myself before. The image of someone cartwheeling and handspringing in full-plate is one that I adore.

And not exactly. There were two primary factors that I'm aware of that were the cause of plate armor's decline. One was the increasing refinement of firearms which were able to pierce full-plate at short range, rendering it in fact a liability as it afforded no protection and at the cost of speed and so on. The second is more complicated.
Feudalism was already declining at a rapidly increasing pace because the Crusades had set in place trade routes that made mercantilism much easier and doable. Over the course of several centuries, this gradually fundamentally changed the distribution of wealth within Europe as freemen and merchants rose in prominence and numbers, which had an economic effect that turned the control of few resources that made feudalism work for the nobility on it's head. Retaining wealth by land rights and gaining in through conquest no longer presented anywhere near the same level of fortune as before, which is the economic system that knights were built upon. This alone threw them into decline and falling in prominence and practicality.

Then, there is the case of mercenaries in Renaissance and Early Modern Europe, one of the most misunderstood and important elements in that situation today. Long before, skill at arms was a hard and time-consuming skill to acquire. Drafted armies of peasants sucked because they often had little time to train them-a lot of countries actually had laws regarding how LONG they could be drafted for, and it actually wasn't very long. Comparatively, hiring professional mercenaries-who supplied their own gear, had years of training and experience and skill and could be hired and discharged quickly was vastly more effective. While they weren't elite as, say, heavy horseman knights in full plate armor, they were certainly [i]quite]/i] good at combat.

This all had an economic effect of making the previous means of having power-controlling a very limited amount of resources-increasingly obsolete. Knights became poorer and poorer while better equipment became more and more available and accessible. Meanwhile, guns were making headway and decreasing full-plate armor's practicality as well.

So, um, yeah. I started that half-a-day ago and then I was pulled into something for the rest of the day and it's late and I'm not sure where I was going with that. Anyways, armor-and-mercenar-rant-ish-thing over.