I can't find a tram related newsgroup.

I've sometimes wondered why modern tramcar makers don't make double deckers.
Yes, modern artics swallow lots of people quickly but they also take up a
lot of space. A double deck artic (with connections at both levels) could
take the same number of people, maybe more, than a single decker in less
length.

I'm guessing there are 2 reasons the main one being that double deckers have
always been a bit of a British oddity. Yes, there have been other places
in the world that used them but these days in particular the market is in
countries where single deck trams are the norm so we get single deckers.
The other reason seems to be loading times.

And, I am aware of the new double deck trams in Hong Kong.