Why Rambam

The Rambam is the only one who collected all of the halachic decisions from the Talmuds (Jerusalem and Babylonian), Sifra, Sifri and from the Geonim -- and from it all layed out in clear, simple Hebrew the entirety of Jewish Law.

He did it in a comprehensive manner, covering not only the "day-to-day" halachot but also those which are less common, including even those which only apply when the Temple exists (may it be speedily, in our days!).

The objection is often raised that "we don't posken like the Rambam". It is true that most Jews do not in fact live according to Rambam's pisqei-halachah. However, it is also true that they do not live according to the Shulchan-Aruch (with or without the Rama), which most do claim to adhere to. The author of the Shulchan-Aruch himself, in his book of responsa "Avqat-Rochel" #32, in response to one who asks whether or not we must force communities who "go by the Rambam" to go instead by the Shulchan-Aruch, writes:

The answer is, who is he whose heart dares to go force communities who hold by the Rambam z"l, to go by any one of the posqim, Rishonim and Aharonim?!? As it says in chapter Quf in `Eruvin: "The law is forever like Beit Hillel. And whoever wants to practice like Beit Shamai, let him do so. By the leniencies of B"Sh and the leniencies of B"H, and by the stringencies of B"Sh and the stringencies of B"H? About such a person, Scripture says: "the fool walks in darkness." Rather: if like B"Sh, (one must practice) according to with their leniencies and their stringencies; if like B"H, (one must practice) according to their leniencies and their stringencies." Isn't it even more so: If (they said this) regarding B"Sh and the halachah does not go according to them, and they said "if like B"Sh, (one must practice) according to their leniencies and their stringencies"? Now the Rambam, is the greatest of all the legal authorities, and all the communities of Erets Yisrael and the Arab-controlled lands and the West practice according to his word, and accepted him upon themselves as their Chief Rabbi; why pressure them to budge from him?"