It really depends on who you are attempting to convince.
There are targeted commercial carts for specific product types /industries that may or may not be better suited for these specific areas. This comes at a cost and most are rather strict as to customizing.
A look into features, future path(roadmap), upgrade cycle frequency, are serious points to consider for an open source solution. If you compare some of these areas between the available cart solutions I think you will find the momentum on the Zen Cart side.
If you based a decision soley upon the number of users, the commercial solutions would be out, and there are other solutions that have more installs then Zen Cart. This I would attribute to a few factors; What was available at the time many of these stores started, the relutance to switch once established and the knowledge of the installers/designers.
As designers are moving to Zen Cart at an increasing rate, the wishes of the clients are still the overriding factor.
IMO for a new store of any complexity there is no contest; Zen Cart wins hands down.