No one wants to commit to a marginal arena with no player base and no upside, simple as that really.
I don't say it's totally wrong, numbers do play a important role... and once an arena is down, it's very hard to get it back on track - but it's just not "simple as that"
Arena population is a result of several factors
- maps
- gameplay & balance
-
freedom of action (including freedom of plane choice)
- numbers on login screen
- novelty factor and even
- placement on the arena lost (as dumb as this may sound at first)
Now the size of influence each factor has is somewhat floating, and they are influencing each other in a complex way. (For example, most maps don't have a large influence, but a few MA maps will make people leave arena in droves).
But in my opinion, the "freedom" factor is the single most important one in the long run, with numbers on login screen the single most important in the short/medium run