Having just been reading about Jan Žižka, apparently he used armoured wagons with small cannons and muskets on them, and they were extremely effective. I also vaguely recall hearing about Da Vinci creating a 'tank' sort of vehicle, which seems like it would have been quite effective. (Assuming it was not hit by a cannonball, it looked as though it could have been almost unbeatable.)
Essentially, what I'm asking is; why are there not more cases of armoured wagons or primitive 'tanks' being used, when their effectiveness had been shown? Did nobody consider that an armoured wagon be effective? Did anybody think of this idea, and deem it unfeasible, or dismiss it for some other reason? (I'm thinking something similar to the Korean turtle ship here, except, on land.)
Quick edit: I'm aware that the horses could be shot. If anybody could mention how Jan Žižka seemed to make this a non-issue, please do explain. Thanks.
There were a variety of so-called 'proto-tanks' in the years leading up to WWI, notably in France and Britain. The one which comes to mind is the Burstyn 'Tank', an armoured vehicle designed by an officer in the Austro-Hungarian Army.
The problem that these designs posed were the same as those of the actual first British tanks: slow, cumbersome, expensive, would require special training and logistical support, and so outlandish that until the onset of Trench Warfare on the Western Front in 1914-15, no one could have possibly found any meaningful use for them.
Assuming it was not hit by a cannonball, it looked as though it could have been almost unbeatable.
I think this may be the real issue here. The Hussite formation was a defensive one. To protect the horses they could be moved within the formation. A ring of static carts would be very vulnerable to cannon fire. Fortunately for the Hussites, cannon in the early 15th century were not that common, and were generally large and unwieldy.
However, by the end of the 100-years war it was becoming common for battles to begin with a cannon bombardment. (France was a leader in artillery.) This was certainly standard practice during the Wars of the Roses. This bombardment was intended to force the enemy out of a defensive stance. The 15th century saw the development of lighter, more mobile and quicker-firing cannon, and of wheeled gun carriages.
So this means an armoured vehicle either needs to be able to withstand cannon fire or be mobile enough to avoid it. If it's moved by horses, those horses need to be protected against small-arms. Those conditions were impossible to meet.
What is worth noting is that where the problem of power and mobility was solved, armoured vehicles did come into use in the 19th century. Armoured trains were used as early as the American Civil War (see an example here). Obviously, using an armoured, steam-powered vehicle on regular roads was even more impractical than using a regular steam-powered vehicle, so this idea had to wait until the internal combustion engine to come into use.
(I've never read anything specifically on the decline of wagon-based warfare, but this post is put together primarily from several books on medieval and early modern warfare.)
Thanks for the answers, gentlemen.