Here's why:

* Diploid cells have two sets of chromosomes (one from each parent).

* The cell cycle is a process that results in cell division.

* Mitosis is the part of the cell cycle where the nucleus divides, producing two genetically identical daughter cells.

* Cytokinesis is the part of the cell cycle where the cytoplasm divides, resulting in two separate cells.

Since mitosis produces two identical daughter cells, and the original cell was diploid, the resulting cells are also diploid.