Mainly because it made lockpicking redundant.
Well, it really makes it so that you can choose whether to handle locked objects via magic or via the lockpicking skill. A bonus for having the costs of spell go way up here is that you could easily do lockpicking without any perk investment. However, if you use spells, you'd need to have some perk investment or equipment to have an affordable cost for the spell -- well, they could make it that way in terms of mana requirements.
Advantage then of lockpicking? You can get more gold, better treasure, etc with its perks (maybe the lockpicking perks would need a buff, maybe not).
In general, I don't see a problem with having a magic way to do things and a mundane way. As long as each has its own benefits and costs to make them more or less balanced with each other.
Overall I don't think theres a big reason we don't have lockpicking via magic with costs of say 100, 300, 600, 900, 1200 mana for novice, apprentice, adept, expert, and master locks. I think those costs would make it fairly prohibitive if you didn't do significant investment in alteration. Well, I would say they should probably get rid of 0 cost spellcasting (cap the reduction from items or make an overall cap like with resistances).