because .....
while i don't have 'designer insider information as to the specific thought process, i can say (based on my experiences in playing as a cleric in almost every single edition of d&d) that there are probably a couple reasons... the most prominent ones being
1) they wanted to stick with vancian spell casting (so there were going to be spell slots)
2) they wanted to make cleric and wizard spells notably different
3) (and most importantly) there was typically a trend, if not expectation, for the cleric to always prepare a lot of cure/heal oriented spells and if he was doing otherwise, he was (at minimum) going to get dirty looks from fellow players so preparing anything other than cure spells never lasted very long. in later editions (i want to say 3.0 but maybe 3.5 or something else) clerics could prepare whatever but had a feature that let them spontaneously swap out any prepared spell for a cure spell of that level. So this is sort of a blend of that in that it lets a cleric still have the option of using other stuff, but the cure spells are still there as an option.
I think you'll notice in the grand scheme that even though the cleric can spontaneously cast from his list, a lot of it will get used in cure spells.
the wizard does not have the same sort of peer pressure that the cleric does. that in itself is a big benefit.
but on top of that, the wizard has more offensive spells (either through control or damage). And the wizard "feel" (at least in previous editions that this rules system is trying to call back to) has always been this 'preplanning and creative use of the spells i decided to pick' sort of caster...
just my random thoughts based on personal experience. YMMV.
