The Sfas Emes (לשנה ה"ד ד"תרמ) writes, "The yomim tovim are called רגלים שלש (literally, 'three feet') because they are the pillars that hold up the world. In earlier generations, three holidays were sufficient. In later generations, Hashem knew that three holidays wouldn’t be enough for us, so Hashem added Chanukah and Purim. About us, it states, נפגע שוטה אין, a fool doesn’t feel.' [We are like fools because we don’t realize just how important Chanukah and Purim are to us.] But the early scholars and the true ovdei Hashem certainly felt and realized that it is impossible to exist without [Chanukah] and without Chanukah lecht, which illuminates the darkness.
"I think we can attain more during these holidays [of Chanukah and Purim than by the three regalim] because we can keep Chanukah and Purim in their entirety. By the other holidays [Pesach, Shavuos, Succos], we lack the mitzvah of לרגל עליה, to go up to the Beis HaMikdash, which was the primary joy of the holiday. But we can keep the mitzvos of Chanukah and Purim fully. We can keep Chanukah and Purim exactly as they were originally established, which is a quality that Chanukah and Purim have over the other yomim tovim due to the Churban Beis HaMikdash."
