One reason is very straightforward. The above picture is one situation where either a gear chain or a gear belt could easily be removed and replaced from the end of a sprocket. But, for sprockets mounted on a shaft between bearings and among cams, a gear chain can be replaced much more easily. (See this for example) With a gear belt, unless a spare was installed during machine manufacture, the only option is to remove the cam shaft - a much larger task.
A gear chain is also more rugged than a rubber gear belt. For example, should a metal guard become loose or damaged and rub against a gear chain, it will make noise and will likely be damaged by the chain. Given a machine running automatically with little or no operator intervention, such a situation would eventually attract attention and cause someone to stop the machine. A gear belt on the other hand will create little or no noise and likely self-destruct before anyone notices the problem. Such a scenario, though remote, can and has happened -- to other machines. A machine that needs a guard replaced is an attractive alternative to one that has crashed in need of thousands of dollars of repair labor and is 'down' for weeks of lost production.