One of the motive in Stoker's Dracula is the travel between Western and Eastern world. It's highlighted many times in the book, especially in the beginning, that the main protagonist is leaving the land of civilization and is coming to the land of wonders, magic, in short he enters Orient. The reason why he chooses Transylvania over Wallachia is maybe because Transylvania was a part of Austria-Hungary at the time, so the transition between West and East was more evident. When the hero was leaving Budapest he said:

The impression I had was that we were leaving the West and entering the East; the most western of splendid bridges over the Danube, which is here of noble width and depth, took us among the traditions of Turkish rule

He described Transylvania as a land of mostly unkown history, he couldn't even find the map of this country. He also mentions that land is inhabited by the descendants of Huns, Romans, Saxons and Magyars. He is eating all sort of strange dishes. The people he sees while traveling to Dracula's castle are becoming to him more and more "queer":

The strangest figures we saw were the Slovaks, who were more barbarian than the rest

Later he said that he:

read that every known superstition in the world is gathered into the horseshoe of the Carpathians, as if it were the centre of some sort of imaginative whirlpool; if so my stay may be very interesting

Author deliberately creates the image of the most oriental place in the country that is still considered part of western civilization, so the business travel of Jonathan Harker and expected move of Dracula to London may seem more propable. Moreover the castle of Count Dracula is placed in the most farthest place in the whole region, at the very border between Transylvania, Moldavia and Bukovina.

This is a difficult one to answer. A number of valid things have already been said but we are still speculating as we don't have a clear answer from the author.

That said, the connection between Vlad the Impaler and Dracula is a weaker one that generally thought. Elizabeth Miller is an author arguing that Stoker might have known very little about Vlad the Impaler, maybe only his nickname of Dracula. Thus the location of the count/castle is a matter of Stoker's choice. Here is a fragment from an article talking about this and about the location.

"Through Stoker's notes, we can also see how the novel changed over time. Originally, the count wasn't from Transylvania at all; he was from Styria in Austria. And it appears that before he came across the name Dracula, Stoker was calling his vampire "Count Wampyr." (The pages that reference "Count Wampyr" are undated, but in some places, Stoker crosses out "Wampyr" and replaces it with "Dracula.") Like so many fictional characters, Count Dracula is an amalgam, a mix of information that Stoker found interesting and ideas he developed along the way. Dracula isn't even representative of one Eastern European state; he's a pinch of Transylvanian folklore (filtered through English writers) with a Wallachian name and a feudal estate straight out of English Gothic literature." Souce: http://io9.gizmodo.com/no-bram-stoker-did-not-model-dracula-on-vlad-the-impal-1648969679

I would also add that besides placing Dracula in a different region than Vlad the Impaler, the Stoker also has Dracula say at some point that he is a Székely, which is a of course a different ethnic group that the one Vlad the Impaler was a part of (Romanian/Wallachian). https://en.wikipedia.org/wiki/Sz%C3%A9kelys

Clearly Dracula is a work of fiction, not a historical account.