Knights wore full plate armor for tournament jousting because the battlers simply had to charge in one direction and see clearly enough to point.
At the same time, their squires could rush out and attend to them once they were knocked on their breastplates or cruppers.
However, if a squire were to help scoop his fallen knight out of the mud in battle he surely would be vulnerable—probably killed on the spot by the enemy.
Most squires back in the Middle Ages found it safer to simply run for the hills than stick around to rescue their fallen knights, and so plate mail fell out of favor.
Besides, it became more and more difficult to produce usable armor that was also thick enough to keep those newfangled gun bullets from penetrating.
The answer is that there were two kinds of full plate armor: jousting armor and battle armor. Battle armor was highly practical and worked very well, to the point that fighting a man in plate required whole new combat techniques. The idea that one cannot stand up or mount a horse while wearing it is a 19th-century myth. It is possible to do quite acrobatic things in full plate.
Jousting armor evolved in a different direction. Since the jouster’s only concerns were withstanding the impact of his opponent’s lance and guiding his own lance home, there was naturally a trend toward heavier, more restrictive armor.
You can see a comparable trend in modern fencing weapons, which would be utterly impractical in real combat–too light to deliver a deadly cut, too flexible to stab effectively. Fencing stopped being a fighting art many years ago and became a sport, and fencing implements changed to match. Jousting followed a similar pattern.
(Also, plate armor survived the advent of firearms quite well. That’s where the word “bulletproof” comes from; the armorsmith would fire a pistol at a completed breastplate, and the resulting dent was the “bullet proof,” demonstrating to the buyer that the armor could withstand a pistol shot. The main reason for the decline of plate was the growing effectiveness of cheap, lightly equipped infantry, combined with the rise of large, state-sponsored professional armies. When the individual knight was paying for his own gear, he would of course spring for the best armor he could afford; but when the state was paying for it, the cost-benefit analysis became about “What will get the best results for the money?” rather than “What will keep me personally alive?”)
Another reason for plate falling out of favor: As guns improved (comparing, say, an early arquebus with a late Renaissance musket), plate had to become thicker and thicker, until it became prohibitively expensive and heavy.
Also, I am kind of irked whenever video games portray maille as being more mobile than plate. While it might have been lighter (depending on the maille and the plate it’s being compared to), it required much thicker padding than plate, making it uncomfortable and poorly balanced. Coupled with its near-inability to protect against blunt weapons such as maces, maille was almost never worn in full suits (with some exceptions, such as the infamous Cataphracts of Western Eurasia), and was mainly used to protect against missile weapons and to protect the joints of plate armor.
Also, the article uses the term “plate mail”, which is not a real thing; it is frequently (mis)used to refer to plate, but the closest thing is “plated maille”, plain old maille with embedded metal plates.