Because the adjective "historiske" is there preventing the definite article from being attached to the end of "centrum", which is apparently a neuter word, so it's "det" instead of "den". Hope that helps (:
When a noun is modified by an adjective, you can't use the definite form of that noun. Instead you use the definite article before the adjective. In this case, "centrumet" becomes "det historiske centrum".