Why is believing astrology a sin in Islam?

Because we believe in Qadr.

Because Destiny does not depend on the constellation of luminaries , but rather we believe in Qadr the destination that has been decreed.
Peace

[Edit]
Christian response, not to override or debunk the Islamic responses above.
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Islam, Christianity, and Judaism all have the same 10 commandments.
The first commandment in each case is, “You shall have no other gods.”
Many religions, including Christianity, view astrology as worship to a false god or false profit.
Its not unique to Islam, were just more lienient in the states.

Assalamualaikum
Because it is a form of fortune telling and sorcery and such stuff is explicitly prohibited in the Glorious Quran.
Muslims believe in destiny and fate, not what the stars have to say.
“O ye who believe! Intoxicants and gambling, (dedication of) stones, and (divination by) arrows, are an abomination,- of Satan’s handwork: eschew such (abomination), that ye may prosper.”
(Glorious Quran 5:90 http://www.allahsquran.com/read/verses.php?ch=5&vid= )
And Allah knows best
May Allah provide us with guidance
Salaam

because its similar to fortune,and tells u in future
Salam
=)

There are narrations of the Prophet regarding astrology. Astrology is considered Satanic knowledge. It is full of lies. It might contain some truths too but you have no way of knowing which part is truth and which is a lie. That’s one of the tricks Satan uses to get people.

It is shirk!
“Say: None in the heavens and the earth knows the Ghayb (Unseen) except Allaah”[al-Naml 29:65]
al-Bukhaari (801) and Muslim (104) from Zayd ibn Khaalid al-Juhani who said: “The Messenger of Allaah SAWs (peace and blessings of Allaah be upon him) led us in praying Fajr at al-Hudaybiyah, following rainfall during the night. When he had finished praying, he turned to the people and said, ‘Do you know what your Lord has said?’ They said, ‘Allaah and His Messenger know best.’ He said, ‘This morning one of My slaves became a believer in Me and one became a disbeliever. As for the one who said, “We have been given rain by virtue of Allaah and His mercy,” he is a believer in Me and a disbeliever in the stars. But as for the one who said, “We have been given rain by such and such a star,” he is a disbeliever in Me and a believer in the stars.’” So the one who attributes rain to the stars is saying that the stars caused the rain.
narrated by Abu Dawood in his Sunan with a saheeh isnaad from Ibn ‘Abbaas (may Allaah be pleased with him), that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever learns anything of astrology has learned a branch of witchcraft (al-sihr)…”
al-Bazzaar narrated with a jayyid isnaad from ‘Imraan ibn Husayn that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “He is not one of us who practises augury or has it done for him, who tells fortunes or has his fortune told, or who practises witchcraft or has that done for him.”