Based on my extrapolation based on steamspy data and capcom data. Atleast more than a million SFV copies were from PS4. So why did SFV flop on PC?  Is it competition,negative word of mouth or the PS4 is just a beast and that games will still sell despite being a critical flop or it being niche?

Yes, but PC is becoming viable outlet for Japanese games. I think two factors are in play. 1. It is SFV, and honestly don't know if fighting games really translate perfectly at times to the PC. 2. SFV wasn't the most well recieved product, generally PC gamers get to see that infront of there face when making purchase decisions. 

Lack of content at release, and way more options on PC rather than PS4. Plus, if they have to wait, they might as well they wait for a discount.

There is a diffirence between selling less but still respectable sales on PC and flopping on it entirely.

So much has it sold on PC?

Too expensive on PC for a half baked, Non-Early Access game still months after release.

Street Fighter V Wasn't a Flop the fanbase for fighting games in PC isn't so big, Street Fighter had nearly the same launch as Mortal Kombat X, but it sales slowed down after that because of his Launch, it need  to sell 500 Thousands in 1 Year to surpass Mortal Kombat X.

I Think that Street Fighter V debuted with 105-120 Thousands, and Mortal Kombat X with 140-160 Thousands, for the PC.

Your extrapolation? :--P

Capcom says 1.4M shipped + digitally sold, Steamspy has it at ~150k, so we know the other 1.25M are on the PS4.

Also Street Fighter V isn't a Niche tittle neither a Flop the game had a Terrible launch and it affected the sales, even so they Shipped 1.4 Millions (Counting Digital Sales) from the Secound half of February to March 31, Today the number is bigger or atleast the Shipped is actually sold throught.

SFV got destroyed by the media/word of mouth. IMO, the average PC gamer is more informed than the average console gamer, which caused less impulse buyers.