Throwing lances are scary, but then, you have to dedicate your entire build to using them and you can only possibly carry 4. It's like an archer with super deadly arrows, but only 1 per quiver and it's a 7 PD bow. Not only do you have to screw your character over to access it but then you hardly get enough ammo to bother with.
And while the throwing spear-type weapons are all relatively dangerous, they are super hard to master because, I think, you get too little ammo and they're damn hard to see anyway.
An archer can eventually learn to get good because he probably fires an average of 20 shots per round. More if he doesn't get lanced early on. A thrower probably throws an average of 8 weapons per round. It's just harder to get experience with them.
But throwing axes are big and slow. I think they're much easier to see and therefore get a feel for and you're more likely to recover them for more throws.
Plus there's just so much more comedic value to hitting someone in the head with a throwing axe rather than a spear of some sort. I can't put my finger on why, exactly, but it's at least 10x funnier.
And (AND!) that heirloom mod makes +3 heavy throwing axes look great and they really stand out on the ground against the other, lesser axes. I used to suggest for people to get +3 gloves before anything else because it's good bang for the buck. My new advice is +3 heavy throwing axes. Unless maybe you plan to be a spearman or something. Throwing + unsheathable weapons is kind of a pain.