I'd say primarily because most people who are new to Linux start with Ubuntu. That makes it popular, but it also makes it a clear "starting point". When you ask for recommendations on a distro, most experienced Linux users are going to assume you've used Ubuntu already. If you make it clear that you haven't used Ubuntu, most experienced users are going to tell you to go with a distro that's a starting point, but also something you can grow into. Ubuntu lost that a while back (people are mentioning snaps, amazon searches, unity, etc. and they're all right) because they started trying to make the OS what they think you want instead of embracing what makes Linux popular, which is the ability to make it into whatever you want.

Also worth noting that Ubuntu isn't a very enterprise friendly distro. Granted, most users won't really care about that, but as you get deeper into Linux, you start to want certain capabilities. Distro's like Fedora/RedHat, or Suse have a steeper learning curve, but you can easily make them as UI friendly as Ubuntu, and in addition to that, they are generally easier to integrate into existing infrastructures. i.e. Joining a domain is possible in Ubuntu, but it's much easier in Fedora.

To really answer your question, think about it like cars. Lot's of people drive Hyundai's, but very few people recommend them. You're more likely to hear someone say "I used to drive a Hyundai, but now that I drive a Subaru, I'll never go back". I'm not a car guy, so please don't judge the choice of brands in this analogy.

TL;DR - Ubuntu is for novices. Once you ask for recommendations, you're going to get advice based on the experience of people who have moved past novice level.