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there's a few reasons

-most hdd brackets come with screw holes or metal pins on the side not at the bottom..(so the standard is side mounting not bottom mount)
So many people are used to mounting the hdd with side screws/pins, not bottom screws

-in terms of ease of use,
you can quickly just pull out the hdd and place on a flat surface and immediately put on or remove the screws when it is on the side
instead of need to turn it over and turn it back when the screws on the bottom etc..

-screws on side of hdd are better placed for all hdd ie at each corner, more balance support for the hdd.

-screw holes on the bottom of hdd,
not all hdd have the holes at the each corner..
some hdd have the hole in 2 corners + 2 middle section of the hdd so it is not as balanced..

-you don't need to add so many screw holes on the bottom placement like your model (ie 8 for hdd)
when you place holes on the side, you only really need 4 holes for most hdd..
you can optionally add the two hole on the side (in middle) so total is 6, but not needed
since most just mount with 4 holes at the 4 corners on the side of the hdd.

I could mod this myself if I had the original model
but I don't like editing the original STL mesh file..
It is not too hard or take too long to place 4 holes on the side after getting the measurements
so up to you if you want to add the alternate version