A timing belt is a very precise way to time engine parts that require it as evidenced by the successful use by VW for many years to operate diesel fuel injection pumps, the timing of which which is critical to good engine performance and fuel mileage.
Another reason for the trend to timing chains from belts is that consumers have become more aware of the expense of changing a timing belt that can be avoided with a timing chain. As evidence for that, during the past couple of years I have seen many posts here on CarTalk questioning the need for and the expense of changing a timing belt.
Timing gears have been used for flathead, OHV and even an OHC engine in at least one Honda motorcycle but the distance from the crank to an overhead cam is too large for just a pair of gears so several gears would be needed with consequent loss of timing accuracy due to backlash. Harley Sportster OHV engines use several valve timing gears, one for each valve that are manually selected for backlash to keep the gear noise down.