Basically it's because the Bible is so obscure and such a conglomeration if ideas from multiple sources. Nobody can really be sure exactly what it means.
Take a verse like, "Thou art Peter and upon this rock will I build my church..."
The Catholics (who opportunistically claimed Peter as the first Pope) say that in this verse Christ is handing them the keys to the kingdom.
Protestants point out that in the Greek (though presumably Christ was speaking either Hebrew or Aramaic) it reads, "Thou art Petros (small rock, pebble) and upon this Petra (large rock, crag) will I build my church." They think Christ was saying, "You're pretty good but it is upon myself that my church will be built."
Now multiply that kind of conflicted interpretation right across the New Testament and you've got people rushing off all over the place to start another denomination.
But perhaps the best way to resolve the dissension is a quote from Hinduism's Rig Veda: "By whatever name you call me, it is I who will answer."
Of course, I can think of a few "true" Christians who would call that Devil talk.
Because people decided that the correct path was too difficult, and so they decided to make their own path, which was much easier to follow, and much more convenient. Which is why we now have ridiculous doctrines such as Sola Fides, Sola Scriptura, The idea that the Ten Commandments don't really matter at all, and the like.
Because there have been, and still are, too many versions of the books called gospels.
What is the foundation of bible? Let us see. There were too many manuscripts containing portions of the NT. They are all written in Greek. No two of these manuscripts agree. Nobody knows who wrote these manuscripts. The disciples of Jesus, so far as we know, knew only Hebrew.
Nobody ever saw anybody, who had seen anybody, who had heard of anybody, that had ever seen anybody, that had ever eye-witnessed one of the original Hebrew manuscripts. These manuscripts are written in what are called capital Greek letters. They are called Uncial manuscripts, and the New Testament was not divided into chapters and verses, even, until the year of grace 1551. In the original the manuscripts and gospels are signed by nobody. The epistles are addressed to nobody; and they are signed by the same nobody. All the addresses, all the pretended ear-marks showing to whom they were written, and by whom they were written, are simply interpolations, and everyone who studied the gospels has derived his own interpretation, which subsequently led to own creed and then to so many denominations.
The Bible holds the Truth! Many prodistants(sp) churches have started by not seeing eye to eye over simple things as dress/ baptism/. These are the basics: The Bible says. ONE God . ONE son. ONE way to God(thru Jesus) That Jesus died- buried- rose again-! Jesus died to pay for our sins by His being that perfect sacrifice of blood! The perfect Lamb of God! Belief in the Holy Spirit that comes into you and seals you until the day of redemption! Salvation is a free gift from God/not of works!( but we work because of our faith- to do as Christ taught) Obey the cammandments ( as close as you humanly can) Fellowship with like believers! That is Gods truth! Look for these teachings at the churches you visit!
Christian people attend different churches and denominations, mostly because they are comfortable with the people they meet there, the type of preaching, or the style of worship. Different churches offer lots of variety in music and worship styles and this is fine. But all denominations that claim to be Christian must be united in one thing-truth. The only source of truth is the Bible, God's Word to us.