Tall cat trees are usually built from several posts and platforms bolted together, and they rarely fit through a doorway fully assembled. Taking it apart at the same joints it was built from is faster and safer than forcing it through sideways.
Pro tip: Save the manufacturer's original assembly instructions in your phone's camera roll before you get rid of the box; they make reassembly much faster.
Watch out: Forcing a fully assembled cat tree through a standard doorway can crack platforms or snap support posts; disassemble first if it is taller than about five feet.