I can only assume they had bad memories.
All those disciples, bishops and other assorted Christians sitting around a table trying to write The Bible [or “Christianity for Dummies” as one of them wanted to call it], and not one of them could remember the date of the Resurrection?
They even argued the toss for a few weeks trying to decide which day of the week it had been before deciding it was a Sunday.
But which Sunday?
They remembered that it was Spring all right as they had had to change their clocks and watches beforehand, but that only narrowed it down to a Sunday after the Spring Equinox.
Then someone remembered that there had been a full moon a few days beforehand as he had been out with his missus one night and she had gotten all romantic.
And so it was decided.
Easter was the first Sunday after the first full moon after the Spring Equinox.
Much more memorable than a simple date?