To further explain and repeat what was said elsewhere, its very easy to find a picture of something cool as well as a wiki page and then say "why cant/haven't you implement it?"
For a vehicle to be added, the model artists need reference photos, drawings, blueprints, images and dimensions just to begin. Then there is a need for historical sources, performance, armor, weaponry, capabilities all backed up by credible material, not just a singular website or wiki. Lots of vehicles are indeed in existence that would make for very interesting event vehicles, but then there is the consideration of are they actually viable to implement? Can all the material actually be found and located?
The other thing to consider is as we have already said, a nation like France or Italy does not have the near infinite number of vehicles that can actually be added to the game. Speaking realistically about machines that actually existed in proper form with the information behind it to implement, every time an event vehicle is added for a smaller nation, thats one less vehicle that could have gone in the tree as a researchable vehicle of regular premium. Even vehicles are rare collectable machines intended to be in short numbers. So every time one is added for France for example, thats one less machine that new players or anyone who doesn't take part in the event / buys on the market afterwards can actually obtain easily.
Events also need to be interesting for the largest number of people. Sure, you can probably find some random obscure tank nobody has ever heard of and doesn't care about, but then who is going to want it in events other than the most dedicated collectors and the 5 people that have heard of it?
These are all factors that need to be considered when making "but there are 500 machines that could be event vehicles!" idea is put forward.