Some korbanos require n'sachim (accompanying flour, oil, and wine) and some do not: specifically, all korbanos require n'sachim except the b'chor, the maasar b'hema, the pesach, and most chataos and ashamos. (Source, mishna, M'nachos daf 90.) Why do the members of the one class of korbanos require n'sachim while the members of the other do not? (I'm not asking how we know the difference: there are p'sukim from which each case is derived. I'm asking why the difference exists.)
According to R' Hirsch, each element of the korbanot is meant to express a particular emotion or intention on the bringer's behalf. Pertinent to this issue:
R' Hirsch says that the obligation to append nesachim of these last three elements to korbanot 'ola and shelamim is a lesson to the bringer: "it is not only the nefesh, the thinking, feeling, striving, active life of a person which ... belongs to God and His Torah, but ... also all ... the elements which are necessary for his existence, his well-being, and his joy in life are no less belonging to God and His holy Torah, come from Him, are dependent on nothing else but Him, and can only be gained by us by fulfilling His Will by "keeping godliness alive on earth".
With this foundation, R' Hirsch explains why this obligation of nesachim applies to some korbanot and not others.
They don't apply to korbenot chatat and asham because "sin and consciousness of guilt which are expressed in chatat and asham are not compatible with the happy bliss that finds its expression in nesech.
In addition (see commentary on v. 3), since the point of this extra obligation is specifically to teach an essential lesson to someone who is voluntarily dedicating himself or his happiness, it applies only to categories of korbanot that can be brought as voluntary offerings, which excludes bechor, ma'aser, pesach, chatat, and asham. (The nation is required to dedicate itself regularly to God in a similar manner through the 'olot tamid and musaf, and this dedication of the national self similarly includes a dedication of all the nation's possessions. See commentary on Ex. 29:38-40.)
The sole exception to these rules is the set of korbanot, including a chatat and an asham, brought by the metzora', which do require accompanying nesachim. R' Hirsch explains in his commentary to Lev. 14:10, which prescribes these korbanot for the metzora', that the point of these korbanot is to express his stripping himself entirely of the traits of self-centeredness that caused his tzara'at and exile from the community. Therefore, "more than with anybody else, just with him, at his now complete re-entry into the Jewish Community, is the fact to be impressed that his future 'flour, oil and wine', his future existence, well-being and joy in life are solely dependetn hin his ordering his life on faithfulness to duty."