Because I cant see what to cut. I need that ramp spells to get to 8+ mana for the higher cost cards and if I cut two of the higher cost cards I could end with 8 mana, but nothing to play except cards that gives mana.
And its a little in favor of Moonlit Guidance, but it was close because Nature Studies sometimes got me a spell that weren't in my deck, that helped me win the game. Maybe I should play with one of each. :)
Studies is an insane card for druid, it helps speed up ramp, it helps find the ramp. I just don't see a ramp druid without the card, even if wild dilutes its pool a bit. The only question is, should Moonlit Guidance be included? It's great if you need value to win, but not so great otherwise. Does it help much against priest? Against aggressive decks, that tend to be an issue for ramp druid? Possibly not. So unless you play an incredibly slow, value based meta for some reason, you don't really need it.
Always Moonlit Guidance, especially in a ramp druid deck - the value you get from it is just insane and the chance to get something useful out of it (where the odds are higher cause it is a card from your deck and not randomly generated cards like in the Studies) is much bigger. The only druid deck where I might consider Nature Studies over Moonlit Guidance would be in an Aggro Druid deck, but even there I would rather cut the Studies than Guidance.