Well Luna couldn't do anything because she was trapped in the moon just prior to this castle's abandonment. So that just leaves why Celestia left things here and honestly, it's pretty easy to imagine why. Not only did Celestia leave the Library (with Celestia and Luna's personal books and artifacts) she left the Elements here too. Why? Because this was the place her sister was corrupted by the Nightmare. Where she fought her sister. Where she banished her. She never wanted to come back here and didn't want anyone else to either. This is a horrifying and terrible place for her.



You mention repeatedly that "a baby dragon" could get here but really the forest wasn't that safe for the past millenia. There were Manticore, Basilisk, Hydra, Timber Wolves, ect... Spike only really went in after the forest's dangers had been tamed significantly by the Tree of Harmony getting its power back (not to mention the Mane 6 and Zecora dealing with more than a few of them). No pony wanted to enter anyway as it's "not natural" and there aren't exactly wandering super-villains roaming the countryside purposely going into strange and deadly forests on the off chance magical artifacts might be there.



It doesn't make Luna look like an idiot, she could do nothing at all. It doesn't make Celestia look like an idiot either, it makes her look like an emotionally damaged and grief stricken ruler. Someone with far too much on her plate (you know with controlling both the sun and moon, ruling a kingdom that had just lost one of its rulers resulting in unknown damage, and so on) and no desire to revisit a ruin that no one was going to mess with anyway.