This is speculation...
A. Christianity, Judaism, and Islam are the three major religons of the world. All three claim to believe in One God and believe that they received revelations from that God. As a result, the majority of the world's inhabitants adheres to one of these three faiths. As Allah Subhanu wa Ta'ala did indeed send Books to all three of these denominations, He addressed them specifically. Now in the Quran, Muslims are addressed, and Jews and Christians are referred to. The rest are considered polytheists.
B. Sikhism was founded by an individual who created the religion. It's actually a mixture of Hinduism and Islam. This is an artificial, man-made faith. There are dozens of religions like this being developed every day. Allah Subhanu wa Ta'ala is addressing the faiths that HE sent Prophets and Messengers to. The one for which HE sent revelations. This "tripartite" of faiths is all stemming from belief in the same God. Sikhs are not considered to have belief in the same God as us.
C. Sikhism was founded in the 15th century. That's wayyy after the Quran was revealed. The Quran alludes to polytheism and shirk as many of the world's faiths contain elements of this. However, I believe Sikhism isn't directly addressed as it not only falls int the category of polytheistic faiths, but also because it doesn't stem from the same belief in monotheism that the Abrahamic faiths do.
Hope this makes sense.