Sorry, I started this like, 2 hours ago, at least, but I never got a chance to post it before the coversation probably moved on. I can't tell for sure, I'm on
my phone.
As opposed to lando, Diggersby gets priority, while lando is reliant on its bulk (good, but not incredible when you're 4x weak) or choice scarf to be successful. This isn't necessarily bad, but it is a nuisance. Diggersby, on the other hand, can run LO for even more power. Gliscor has the same problem, albeit more reliant on its bulk, as speed is enough to beat stall, exca relies on sand, and shares common weaknesses w/ Ttar, mamo is the awful ice type (defensively) etc.. You're right that all those have a major niche over digger bunny, and some (actually gliscor, especially) aren't even really similar. Few to none of the mons you named really perform the same role, exactly. Lando is almost exclusively a pivot or wall/ rocks layer, gliscor either stalls or breaks stall, exca can perform multiple roles; a SR sweeper, hazard control, or potent scarfer, garchomp is usually a bulky rocky helmet set atm, that or sash lead, but rarely if ever a straight up sweeper anymore, you get the jist. The most similar is probably mamo, as they actually have basically comparable stats, but mamo has a better offensive typing, a worse defensive one by far, slightly more speed, significantly less power, weaker stab, no swords dance, and the ability to go (kinda) mixed w/ freeze dry.
Also, I think diggerbunny can catch the opponent off guard, like they underestimate it. Lando t, for example, is immeadiately noticed and many teams will target it as it's an obvious threat. I don't feel the same is true for digby. It can kinda be effectively used as a "secret weapon" to just rip entire teams apart.