Why is everyone assured of a place in the Resurrection and World to Come, but not everyone will necessarily enter Gan Eden?
Gan Eden (the Garden of Eden) is the destination for souls after death, while the World to Come is the ultimate reward for the body and soul together. Gan Eden is the reward for Torah study, which is done primarily through the intellectual power of the soul, not by the physical body. Therefore, only souls without bodies will enter Gan Eden. A soul that studies Torah in this world will merit to enter Gan Eden and ascend to higher and higher levels, understanding the Torah in an ever-deepening manner.
Torah can be studied on many different levels. People differ in their intellectual ability; some can only learn on a simple level while others are world-class scholars. Someone who has never had the opportunity to study Torah—through no fault of his own—nevertheless has no “passport” to enter Gan Eden.
The Resurrection, on the other hand, is a reward for the fulfillment of Mitzvot. Mitzvot are performed primarily by the physical body, and thus there is less of a difference between one person’s fulfillment of Mitzvot and another’s. True, Mitzvot require intention, and some fulfill Mitzvot with a deeper understanding or greater zeal. However, the main thing is the action, in which we are all equal. For example, a simple person who dons Tefillin has the same Mitzvah as Moses himself. Therefore, our sages say that “All Jews have a share in the World to Come.” All Jews, over the course of their lives, have performed many Mitzvot and thus will surely be rewarded with a share in the World to Come. Therefore, no one will be excluded from the Resurrection.
(Sanhedrin, Perek Chelek. Mesechet Chagiga, end. Sefer Maamarim Melukat, vol. 4, p. 177. See also Likutei Sichos vol. 17, p. 343. Igros Kodesh vol. 1, p. 143 and vol. 2, p. 73)