Why "bookshelf" sspeakers?

One disadvantage of tower speakers is that, depending on your room, the placement that is ideal for the mids and highs may not be ideal for the bass. 

Bookshelf speakers are typically smaller, simpler, and less expensive for a given quality level. At a given price point, less money has to go toward a larger cabinet, and can go toward better drivers and crossover parts. A well designed two-way can have pretty incredible clarity and coherency...there's no high pass capacitor in series with the midbass driver.

I’ve always been a fan of bookshelf/standmount speakers. I heard the comment in the past that a good big ‘un beats a good little ‘un and generally I agree with that, though I still like standmounts for a few reasons. I agree that standmounts with stands taking up pretty much the same amount of floor space, but I find the standmounts can have a smaller visual impact. 
 

I tend to build my own speakers, so I find that smaller speakers are easier to build in my shop. Materials cost is less as well. 
 

As mentioned above, for a given budget it may be possible to go higher up a manufactures line for a smaller speaker, though in some cases the cost of the stands can even out that difference. 
 

Good thing is that there are plenty of options no matter your preference. 

I have stand mount speakers mostly due to the constraints of my listening space.  I have a smallish room and am not far off from near field listening.  The stand mounts made more sense to me than a floor standing speaker.  I thought the floor stander might be too much for my room.  The stand mounts have the ability to disappear as well which may or may not be the case with a larger floor stander, at least in my room.  That said I just purchased a pair of REL subs to fill out the bottom end so maybe it’s all a wash.  The speakers with stands and the subs certainly take up as much room as a floor stander would.