The massive popularity of Iron Router is just because it’s been around longer. That’s what people are familiar with and it will take a while for people to move over to Flow Router (if ever).
I’m very familiar with Iron Router and I use it in my projects, but if somebody just started learning Meteor now I would definitely recommend that they use Flow Router since MDG do and Iron Router reactivity does become a bit of a mess when a project grows.
I’ve never used Flow Router, but at some point in the future I will switch over to it. For new projects I work on I plan to use it. Transferring old projects to Flow Router isn’t worth my time at the moment.