Abarbanel opens the parsha with seven different answers (“b’shiv’ah derachim yanusu mipanecha…”) to the question of why the Torah promises material rewards for the observance of mitzvos and never makes any mention of the spiritual rewards. Rishonim do NOT answer that Biblical Judaism had no concept of reward in an afterlife and these ideas only developed afterwards (I hope you are not surprised). One answer they do give is that the pagan societies had no concept of reward in an afterlife. Avodah zarah “deities” were viewed as being in control of nature. Worshiping them was the means to ensure that one’s crops grew, enough rain fell, etc. Had the Torah only promised an olam ha'ba reward, a farmer might feel his commitment to Torah comes at the expense of the good material life that his oveid avodah zarah neighbor enjoys. Therefore, the Torah promises the same material rewards as avodah zarah worship as well (we are talking gross gain of course, before the high cost of tuition and kosher food sends you into poverty : ) The Torah wants to make it less challenging to leave behind avodah zarah. If you don’t like this approach, the Abarbanel has six others for you.
I think it's worth noting what Abarbanel doesn't ask in addition to what he does. Again, his main problem is ikar chaseir min ha’sefer – why no mention of the "real" reward of olam ha’ba? The Torah is telling us about the appetizer and ignoring the main dish. Achronim go a step further and ask why the appetizer is even on the menu: “schar mitzvah b’hai alma leika” – the ONLY reward that there is for mitzvah performance is reward in olam ha’ba. It's not the absence of a mention of olam ha'ba that bothers them -- it's the mention of olam ha'zeh that is the problem. Why the Abarbanel does not see ask this and address it (though many of his answers cover this issue as well) may be something worth thinking about.
So why are we promised all these great material things if “schar mitzvah” is reserved only for olam ha’ba? Rashi comments that “im bechukosai teileichu…” refers to ameilus ba’Torah – not the learning itself, but the toil involved in the process. Reuvain and Shimon sit and learn the same sugya, but it might take Reuvain 3 days to master it while Shimon grasps it in 3 hours. The material covered is the same, but the measure of toil is very different. “Anu ameilim u’mekablim schar” – we get reward for the toil. Perhaps Rashi means that the rewards of the parsha are payment for ameilus alone, but the reward for the mitzvah itself, the learning itself, comes only in olam ha'ba.
The Sefas Emes (5651) doesn’t so much answer the question as render it moot. Why shouldn’t we be blessed with abundant crops, with all the rain we need, with whatever material goods we need? Why is this considered something extraordinary, a special reward? The answer is simple: because we’ve become accustomed to a world that doesn’t run the way it should. The post-original cheit of Adam world is one that is tainted, one where things are out of kilter, where we suffer droughts and failed crops and all kinds of other tzaros. What Parshas Bechukosai is describing to us is not so much as a reward as simply a return to the way things should be.
How do we get there? Adam’s sin was the desire for da’as, for knowledge. The way back to the perfect pre-sin world is “im bechukosai teileichu,” accepting the idea of chok, of the unknowable. That is not to say that one should disdain intellectualism. The Sefas Emes himself writes here that, “Tzarich adam l’chapeis b’da’ato k’fi hasagas yado.” A person must exercise his mind to its fullest. The point the Sefas Emes is making is that the intellect is just a tool to understand G-d's world, not to be G-d. The sin of Adam was seeking da'as to be "v'heyisem k'Elokim" rather than to be an "oveid Elokim." The hardest thing is to accept and obey even though one does not understand, or one feels things should be different. That's ameilus ba'Torah says the Sefas Emes.
(See Sefas Emes end of 5649 d”h teilechu for another beautiful hesber of this issue).