Why were Edward, Henry and Toby dropped from the steam team?

To make space for diversity, from wikipedia:

Among many other changes, Edward, Henry and Toby have been removed from the Steam Team to make room for Nia and Rebecca

From The Sun

Henry and Edward will get the boot from Tidmouth Sheds to make room for Nia and Rebecca – who will come from the Mainland near Thomas' fictional island of Sodor.

Producers want the two new female characters to have starring roles when they join Emily, who has been the only girl engine among the main characters for around 13 years.

Ian McCue, one of the producers, said: "We have added new female characters before, but I think, with a show like Thomas, they tend to fall into the background after initial introduction.

"What we wanted to do here was really bring these two female characters to the forefront."