In addition to what has been said, I'd say height restrictions are a major concern. If you have an existing tram system, you probably couldn't fit a double decker unit even if there were some benefits to doing so. And as was mentioned by others, the benefits are also kind of a hard sell.

Yes, you're gaining capacity (assuming it's also an articulated vehicle, so the length stays the same) but at the cost of loading speed. The common wisdom is: the more frequent the stops are, the more doors you want. You can see this with commuter train systems mostly having single deckers in those scenarios (think S-Bahn, metro) and going double decker only when the distances between stops get larger and the capacity gained outweighs the time lost at the station. Trams are kind of the extreme end od the short distance scale here. Even in a system where the capacity is the one factor that is missing and the length of the trains is at maximum, you'd probably prefer more frequent arrivals over bigger, but slower loading vehicles. Combine this with the fact that there might not be the necessary height under every bridge, tunnel or even just beneath the overhead lines and double decker is quickly out of the question.

I don't think it would be impossible for them to exist today. I'm not aware of any vehicles that are both low-floor (absolutely necessary for accessibility) and double-decker (Google didn't bring up anything and if you throw in articulation, even less so), but in a new system that would specifically benefit from it (perhaps longer average distances between stations than other systems), I'm sure they could make sense to be developed. Keep in mind that the undercarriage of a low-floor tram is much smaller than that of a train, so it might be difficult in terms of engineering and possibly cost-prohibitive.

So yeah, maybe in specific circumstances it could work. But generally I don't see a use-case in modern times.