According to https://en.wikipedia.org/wiki/VT05
the VT05 presented the user with an upper-case only ASCII character display of 20 rows by 72 columns.
At first glance this seems a reasonably natural resolution, a little less than the later 80×24, but it feels natural that the very first model should have lower resolution.
But fridge logic: where do the numbers come from? 72×20 = 1440, which is not close to a round number in binary, suggesting that it's not the sheer cost of memory that's the limiting factor. It's also not (or at least, not only) the achievable pixel clock rate; that would limit horizontal resolution but not vertical.
Apparently this terminal used shift registers for its memory, which might be a clue. Did those come in some sort of oddball size that made 72×20 a reasonable fit, somehow?