I can think of two reasonable ones. The first would have to be cost. The belt has to be cheaper than the chain in general. Second....the belt runs in a dry environment, and hence doesn't require any lubrication. The obvious drawback is the periodic changeouts of the belt,, typically 60K miles. Since most OHC belt driven transverse engines are not interference engines (I don't know of any these days....) the risk of bending valves when and if the belt snaps is nil. So probably a combination of cost and not having to include lubrication in the basic design.
And although most shops charge typically $150-250 or so to put in a new belt.....it really is pretty easy on a SOHC engine.
Some of the painful engines i.e. like the 89-94 Talon/laser/mitsu DOHC interference engines... actually have 6 separate belts on the cam drive side of the engine, i.e. timimg belt, oil pump belt, balance shaft belt, a/c belt, p/s belt and alternator belt. This is an exception..compared to say a SOHC engine, which can be changed in an hour or so.
But if I were the design guy,,,,I would go for the chain and make the beancounters include the cost. Less maintenance. I personally would pay for the difference as a customer.