The reason is theological. Observe the following:
We have seen that, from the point of view of Islam, the limitation of Christianity is in having to accept the notion, first, that man is totally corrupted by sin and, second, that none but Christ can deliver him from it; and, as we have likewise mentioned, Islam bases itself upon the axiom of the unalterable deiformity of man: there is in him something which, participating as it does in the Absolute—otherwise man would not be man—permits salvation provided he possesses the necessary knowledge, and this is precisely what is provided by Revelation; what man stands in absolute need of is not therefore a specific Revealer, but Revelation as such, that is, Revelation considered from the point of view of its essential and invariable content. And this crucial point could also be brought up: what Islam blames Christianity for—but not the Gospels—is not that it should admit a trinity within God, but that it should place this trinity on the same level as the Divine Unity; not that it should attribute to God a ternary aspect, but that it should define God as triune, which amounts to saying either that the Absolute is triple or else that God is not the Absolute.
The quote is from an article by Frithjof Schuon, it can be found here: Form and Substance in the Religions.
The context for the paragraph has to do with some contextual misunderstandings between various world religions (basically Islam and Christianity), and how some partial dogmas can be misunderstood for "absolute" statements (each religion has its way of framing the world and its path to salvation). Claims made by certain religions can be understood only in context, but when made absolute they are taken out of context.
At the end of the paragraph, Schuon talks about the trinity in Christianity. Islam does not disagree with stating that God might have a "ternary aspect" (and there are forms of Islam that admit this, like the Alawites), but it disagrees with placing the trinity in place of the Divine Unity, which amounts to saying that God is not the Absolute (since God is triune, i.e. God would not be Himself; "the Oneness").
The footnote says this:
 It is true that God as creator, revealer, and savior is not to be identified with the Absolute as such; it is likewise true that God in Himself, in the full depth of His reality, is not to be reduced to the creative Function.
This is essentially the explanation for why Brahma is not worshiped. While Schuon first elucidates how placing the "Trinity" in place of the Absolute, is sort of like making God "not" the Absolute, he qualifies in the footnote that God is not to be identified absolutely with the Absolute. (For you Hindus, this is the distinction between Brahman and Isvara (i.e. as Siva or Visnu).)
This is the first distinction he makes.
The second distinction he makes is between "God" and the "creative function." The "creative function" is Brahma.
So the reason that Brahma is not worshiped in Hinduism is because God is not solely the creator, or creative function. Identifying God totally with the creative function would amount to worshiping Brahma, which no one does. God as "creator, revealer, and savior" would either be a Saivite or Vaisnava God (i.e. either Visnu or Siva), but the "Absolute" is probably Brahman.
Hope that helps.