From a new-ish, non-tank player, I would say the best way to encourage more tanks is to start early. In low-level dungeons, I had a bit of frustration with losing aggro occasionally, maybe usually only one or two. Or struggling to pick up and add, especially any that are a bit far away, either move the whole pack to try to get it, not ideal, or hope your party can at least think to bring it closer, because trying to pick up from range is (or seemed to me) difficult. The more it happened, the more useless I felt as a tank. Which, idk, could as much be on me and poor tanking skills to begin with.

Admittedly, I could just be lazy. Or just not a good fit for the role. But my impression was the learning curve was a bit steep. I've heard some suggest that things get easier later on, once you pick up tank stance, among other tools for holding things together.

My suggestion for encouraging more tanks is to first ask new players about their impressions. Players that start with a tank and stick with it, players that switch to a tank and enjoy it more, players that outright avoid tanking, and players that tried to tank but gave up. There's always going to be a wide variety of opinions, sure, but it would give a much better idea of how to tweak the role to keep interest. It's a bit unbalanced to go off of an end-game player's opinion when the end-game meta is mostly just "MOAR DPS." A new class isn't going to fix anything if it's the role itself that scares players away.