The reason for so many glen-something whiskies is because ‘glen’ means ‘valley’ in Gaelic.
There are a lot of hills in Scotland, so there are lots of glens. Distilleries are built in glens because:
1. You need a lot of water to make whisky, so the bottle of a valley is an excellent place to be.
2. It’s easier to build a distillery at the bottle of a hill than on top of one.
A lot of distilleries take their name from their location. It isn’t a surprise there are so many scotch whiskies that have glen in their name.