We have שבת הגדול because the נס of crossing the Yarden didn't happen on Shabbos.
The ב"ח (O.Ch.430) writes, "why do we commemorate the נס (sheep tied to bed ) on Shabbos instead of the tenth of Nissan." (actual date of נס)
He explains, "so as not to confuse it with a different נס that also happened on the10th of Nissan. The crossing of the ירדן happened on the tenth of Nissan but it did not happen on a Shabbos.
By commemorating the נס (sheep tied to bed ) always on Shabbos and not on the10th Nissan
no one will think we are celebrating because of the other נס of crossing the ירדן.