In my opinion, these particular models are wildly popular first and foremost because of the relatively immense value they represent. This is especially true of the Monster and the Sumo, as it has sort of become gospel in the watch community (especially amongst Seiko fanatics) that these models are incredible in the bang-for-your-buck department.
Of course, it does go deeper than that. The Monster has what I would consider to be quite an original design, as it doesn't take many cues from the classic diver design. There isn't an ounce of homage in that watch...it's all Seiko, all the way.
Regarding the Sumo, you can barely go a single day without hearing raves about its fit and finish at such a low price point. Having handled the Sumo, I can confirm that indeed it is head and shoulders above the entry level divers (Monster, 007, etc.) in this regard. Also, the Sumo has a bit more exclusivity being that it's a JDM model, and it also has a higher end movement that hacks and hand winds.
In terms of the Tuna, not only is it a great value (or perhaps I should say that it was
a good value
), but it also has a long history and lineage in terms of Seiko professional dive watches that dates all the way back to when these watches were actually used as tools. The watch also features a high-torque movement that was specifically designed for the line (which some might argue qualifies it as an H.E.Q.). Beyond all of that, it too is a JDM Seiko watch, which makes is a bit more exclusive. In my estimation, the Tuna Can is the ultimate Seiko diver, so forgive me if I show a bit of bias toward it.