Why is Genoese replaced Milan faction.

[Nata Aquinta]

In the original virsion it has Milan, but why Stainless Steel not. At least, it must be in the early era.

[Duck of Death]

There is a hard limit on the number of factions you can have in the game (I think around 30 or so).
Youll notice the Stainless Steel map goes quite a bit further east the it did in vanilla.

So the designers wanted to include new factions that they deemed were more relevant on the world stage at that time period, such as Cumans, Kwarezmians and Aragon.
To do that, they had to remove a couple "less important" factions, such as Milan and Ireland.

[Roma_Victrix]

Yeah, the Republic of Genoa during the 12th, 13th, and 14th centuries was the primary opponent of the Republic of Venice throughout the Mediterranean theater. The Duchy of Milan didn't become terribly relevant until roughly the mid-15th century, after the House of Sforza toppled the short-lived Ambrosian Republic in 1450. Even then, the Kingdom of France and Austria dominated the duchy during the early 16th century, followed by the HRE and Spain during the late 16th, 17th, and 18th centuries before it was ceded to Austria (and was finally dissolved by Napoleonic France, which incorporated it into a larger client-state republic).

[Lifthrasir]

^^ Nice input