Because there are 60 billion unique minds in this world. They will be attracted to the different denominations which they feel suit themselves.
God understands this nature especially when modern humans tend more to identify and emphasize what they need instead of what they are told. So save these humans with such a kind of behavior, the tons of denominations are thus needed. The bottom line is, they will only be considered as God's church under certain measurable criteria (not that easy sometimes though). Basically, the Apostle's creed marks such a criteria.
To simply put, God allows the different denominations to attract the different human minds as long as the salvation core messages are conveyed. For those "churches" going beyond the "preaching of the same gospel", which is basically identified by the Nicene Creek and other similar criteria adapted by the existing Apostle's churches, will be identified as apostates/heresies. Actually there are a lot today's Christians are with the capability of discernment to tell them apart.