The Victor Von Doom of the comic books hails from a fictional Eastern European country named Latveria. However, there is already an extremely well-known fictional country in that region that has already been established in the MCU: Sokovia. In addition to being the home country of Scarlet Witch and Quicksilver, it was also the site of the Avengers' last stand against Ultron, and the host country for the Sokovian Accords, legislation that tore the Avengers apart.
In addition to giving Doom a prior relationship with the Avengers (more of that later), it would also clear up confusion for a casual fan. Rather than having two Eastern European countries with historic architecture and similar accents, we'll just have one. Dr. Doom is given an instant connection to previous movies, and no extra effort is required in order to demonstrate what type of country Sokovia is.
As will be pointed out in later entries, events that have already happened in Sokovia will be essential for one of the most major assets of Dr. Doom that has been completely ignored by the three Fantastic 4 movies: his role as the leader of an Eastern European country.