Shabbat is inherently sanctified and established, whereas the festivals are sanctified by Israel who determine the times.

2077. On the Unity of Yaakov and Rachel, Influence of Speech and Action, and Availability of the Rebbe's Maamarim אודות יחוד יעקב ורחל, השפעת הדיבור והמעשה, וזמינות המאמרים שלי