Power shifts in SR are usually pretty slow (it may not feel like it as one buff to one item might suddenly make it so you only see say, 3 adcs, but thats more a reflection of relative power than changes in absolute power), so the tables turning would be a very gradual change for urgot*, perhaps slow enough that no one feels that their achievement was invalidated by any one patches change. I think the system is designed as SR exclusive more to keep people from complaining that their achievement in earning their tokens was cheapened, versus necessarily being the case that other maps actually would cheapen the achievement. That is, it's appeasing player perceptions rather than the reality, as they often do. (e.g. the constant cries of nerf zed/yasuo/lee/etc, compared to something like Janna, who has long been in reality a statistically stronger pick for a team comp and yet never sees bans.)
Also, I don't think they want players venturing beyond SR really. SR is their main game, and everything else is meant to just be a sort of refreshing change of pace on the side that they're willing to support, but SR is still their first and main priority, since thats what new players will be exposed to, what the majority of players will ever seriously play, and what they build league as an esport around. Certainly there are people who almost exclusively play one of those other modes, but they aren't the core audience, and it's a reasonably safe bet that even those players weren't originally drawn to league because of those modes.
There's also figuring out where to draw the line. If one concedes that TT deserves mastery tokens, then why not ARAM? It has a grading system as well, that is in theory appropriately tuned for the meta in that mode. How about <insert RGM here>? Most of those have grading systems too. If they all get mastery token eligibility, then does that throw any design constraints on future modes? Every new mode entirely tends to have a period early on while the grading system (and players themselves) learn and adapt to that mode, where getting S grades is almost easy.
(I might mention I'm not saying that TT and similar shouldn't be eligible, and just putting out thoughts why riot may have made the decision they did regarding token eligibility. It may well be the case that allowing it was [and may still be] the better choice, but I don't think riot made their choice completely arbitrarily.)
*= This is in reality going to be false, since urgot has an upcoming rework, and reworks are massive changes, and notably they don't reset mastery levels when they do reworks. This is a contentious issue.