(This is all just my theory - I don't work attractions, I don't know these attractions' capacities, and I don't know how many FP+ are given out in relation to one another.)
My guess is that Tower's not tier 1 because DHS doesn't seem to 'need' more tier 1 capacity with its current set of attractions.
You can compare Epcot and DHS's current (well, pre-Soarin-refurb) tier 1s fairly easily:
Soarin/Test Track = Toy Story Mania/RNRC (big headliners in high demand - one of each even has single rider)
Illuminations = Fantasmic (nighttime shows)
Living With The Land = Great Movie Ride (in there to take 'leftovers')
(Plus DHS has the BATB show in tier 1 as well - it's not a perfect comparison!)
Now, if Toy Story or RNRC went down for an extended refurb, a la Soarin, I think we might see Tower bumped up into that slot in the interim, especially for RNRC.
W/R/T Test Track not getting dropped down when new stuff opens: While I'm sure the Fro-way attractions will be in high demand once they initially open, I'm not sure they'll see the long-term demand that Soarin and Test Track do, so I don't think they'll displace it from the "big headliners in high demand" category for good. If anything, they'll just bring folks to Epcot who might not have gone otherwise because "there's nothing there for kids." (Which isn't really true, but it's a popular misconception.)