As people point out, Gandhi is a very recognizable figure and probably helps boost sales with the general public.
General public doesn't care in the slightest what historical people are in the video game. General public cares about how cool the video game sounds, looks, what first impression makes and how does it play.
I don't think there are any people unfamiliar with the series, anywhere, who see Civ on the shelf and say 'damn, Gandhi is in - I will buy it'. Besides, I don't think majority of the general public outside India have slightest opinion who Gandhi is, besides 'duhhh I think he was uhm liked love and peace from India'. You greatly overestimate the interest of general public in exotic, faraway history
I'm pretty sure changes of sales would be negligible even if Civ had no leaders at all (well except the fact that's civ tradition - let's just imagine it'd be a 'fresh start') - then it'd be known as 'cool strategy game when you lead great nations through history'.
IMO Ghandi is a good choice because he is funny and when people unfamiliar with the Civ games see him, they have some sense of who he is and what the game is going for. Those are the reasons he is always in the game.
According to this logic every leader for civ should be major recognizable, and every civ should be deeply rooted in popculture, because suddenly people would stop buying the video game if - next to mandatory and classical civilizations and leaders that will always be here - it would have some, even one, more obscure leader or civilization.
I cannot understand this reasoning at all. Furthermore, according to the same logic, if one leader was once chosen by devs to be the leader of a particular civ, it should be its leader for the entire eternity of this franchise 'because he is already known'.
Why there are obscure leaders and civs in recent civ iterations? Because
a) Fresh is fun and keeps the franchise changing
b) General public buys games based on looks, marketing and first impressions, not nitpicky choices of historical figures.
c) There are a lot of super cool characters that are obscure, cooler and more interesting than many popularly known characters, because pop vision of history is terribly deformed. Thereby, Firaxis shouldn't care about it at all, just use the entire human history, more or less known, as an inspiration (also because - point a)
I never really got why some people insist on "right and true" leaders in this series (Hatshepsut for Egypt instead of Cleopatra is another one). I find those "true" leaders boring. IMO the most interesting characters are characters--there's something there to kind of mock. The joke is the ridiculousness of the situation: the Aztecs building nukes while holding off the invading Romans, and cutting deals with the Koreans to backstab Dido of Carthage, who they have learned is building the Eiffel Tower. I mean, this is a game where the USA starts in 4000 BC. It's always been outlandish.
Well, that's the first time ever I meet with the opinion the essence of the franchise is absurd humor, me and a lot of people treat it as monumental strategy game inspired by history and many even view it as 'simulation of history' furthermore, I have never found the base concpets of the franchise 'ridiculous' - it's just deeply alternate history of alternate humanity. So no, I don't think civ should tolerate wild absurd things, unless they are really necessary for the gameplay itself.
Anyway we got Ashoka in Civ IV and he was so dry his name should have been Sham-Wow.
Firstly, Ashoka was fascinating person and Firaxis could fail with depicting him. Secondly, civ4 had incomparable leader's personality and graphics to civ5-6.
Thirdly, I can google two dozens of other great Indian rulers that would be more fresh and interesting than tired, cliched caricature of 'hippie pacifist' that reduces Indian civ in every civ iteration ever to the same boring 'never attack until nuke meme' pattern.
The reason why people oppose gandhi for India is because he is simultaneously boring, overused, inappropriate and unnecessary. Well personally I also find him the most overrated person of 20th century and repulsive, amoral, hypocritical, fanatic fool but not many people actually bother to read his biography or ideology so I'm in the minority