I thought it was kind of implied that Celestia lost the elements a long time ago. Possibly even after having to use them against her own sister. That had to be a real kick in the Loyalty, Laughter, and Kindness, at least. I mean, the physical representations of the elements had turned into stones that were kept in a very decrepit castle in the middle of the Everfree forest. Whereas the repowered elements of Harmony are kept in a locked vault in Canterlot. Which, by the way, kind of backfired on them. Twice. Maybe another solution needs to be found...
Now, if the elements are that powerful one would think that if Celestia could use them, she'd have them in that vault for quick access. But she didn't. Instead she did the only thing she could have, attempt to repower the elements with a new spark of friendship. Which is where Twilight Sparkle and Celestia's mad gamble comes in. So many things could have gone wrong with that, but Celestia trusted her student to get it right, even if Celestia had to be really cryptic about it so she wouldn't contaminate any friendships that were formed. They had to be genuine, not just some goal for Twilight, otherwise, the elements wouldn't trigger at all.
It really was a pretty desperate plan, but it paid off, in spades. Not only did she get her sister back, but when discord returned, well, the Mane Six were sort of ready to face him.
Someone should probably teach Twilight a spell to summon the elements for extreme emergencies, though...