As opposed to watches made in the US or Europe? Aren't most electronics made in China/Japan anyways?
Timex being an American company is less likely both historically and now to sell in say Iran, Iraq or Syria than a Japanese or even a European company.
Historically as far as electronics, cars and similar things go Asian and European brands are more common in the Middle East than American ones.
When you make a detonator, you really, really want it to work.
If you show someone "the wires go here and here" on a specific model you don't need to teach them "real" electronics, and theres less uncertainty that the final product will function.
e.g. Arduino is rarely the best mcu for any given application, but it's popular because a) its easily to source and b) there are plenty of "recipes" to follow which don't require much expertise.
My personal guess as to popularity : volume. Those watches are everywhere.
Is there some reason the watch is "a brilliant way of blowing up explosives" compared to any other watch on the market? Or has the journalist just made that up?