From a designer's standpoint: there can be advantages in a tower (the ease of adding a third driver, or a low-tuned port, or a TL, etc).
These can then often result in the "tower version" of an individual speaker going lower.
But that's all it is: a tendency based on the ease or difficulty of a certain design.
Within reason (I suppose we *could* build a bookshelf with multiple 12" drivers, but it might be unreasonable), there are only two things a bookshelf won't be able to do.
1) Push a very high SPL of a very low Freq. (as that would require a significant surface area); but please never doubt what (for example) a JL 8W7 in a bookshelf can do.
2) Run exceedingly high SPLs (which requires arrays of drivers, making very large towers or (for higher still) complex arrays.
With a sub, in a normal-person room, there's no *inherent* advantage of a tower.
For given money, it is *likely* that the tower you can buy will go lower than the bookshelf you can buy.