Because the fanbase hasnt increased.  If its exclusive, they would buy the console to play it, if its not exclusive, they just buy the game on their preferred console. 

Franchise fatique: This isn't Call of Duty, Assassin's Creed, FIFA etc. There was a 7 year gap between mainline titles. That is almost an entire console generation.

Unnecessary entry to the saga: With that logic,  Zelda, Halo, Uncharted etc would all be selling poorly as they have done little to progress their respective sagas. We know that is not the case.

Waiting for Christmas.... 4 months away.... Don't see it happening. This isn't a holiday people are booking.

Boycotting Konami... Aside from the hardcore Konami/MGS/Kojima fans, I don't see this being valid. Certainly not the the extent where it would affect it significantly.

Boycotting Kojima... This is possible especially considering how bad I thought MGS4 was in terms of gameplay. But like above, I don't see it killing sales significantly.

Piracy.. Is there a mod chip available for the X1 and PS4 that allows you to connect online that I don't know about?

Used games... Have been around since the dawn of time. Brand new AAA games are actually cheaper now than back in the SNES/Genesis/PS1 era. Very little has changed.

Too expensive for its value... Over 100 hours of gameplay in the single player alone. Yeh this is that is the next The Order 1886 right there.

Not having a go at you personally, but I don't see these factor being the reasons why. But alas, I don't know why... You could be right and I just don't see it... How many PS3's were sold by the time MGS4 came out?

Well, that's one, highly subjective way of looking at it. The other is that going multi platform prevented it from tanking all together.

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Well, sales for mgs5 looks better than we thought and there is a slight increase in fanbase.  

Would that not just affect the hardware sales though? Given the actual userbase(PC, PS3, Xbox360, X1 and PS4) would total a much greater quantity than just the PS3 alone in June of 2008, surely the rave reviews and promotion should have attracted buyers?

Yeap... subjective but I believe it could be sold more being PS4 only.

I'm not one to put too much stock in adding digital but it really is growing, you gotta remember. Plus with Sony saying September was the PS store's highest grossing month ever who knows what the digital ratio was for some of these new games, including MGS5. Was MGS4 even released digitally at first? I remember a lot of early PS3 games only got released at retail (some had digital versions added later on, and I know MGS4 is available digitally)

Nvm question already answered, it wasn't. Well there you go.

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Sure. Thing is, the highest selling Metal Gear Solid game is, precisely, the one that has been featured in most platforms. Blaming Xbox here for the sales of Phantom Pain doesn't work.