So looking at the numbers on vgchartz

Infamous

2013: 183k

2014: 25k

Infamous 2

2013: 148k

2014: 14k

Despite being 2 years older, it's has not just shown stronger legs over a similar period of time, but it actually outselling it's sequel, right now! Is it the result of Infamous 2 being free on PS Plus? Bundles? Any other theories?

First big game for PS4. Think MotorStorm and the original Resistance. High attach rates. That is why releasing New ip's are safer at the beginning of a gen rather than late in the gen. Less competition

EDIT: Youre not talking about Second Son.  So idk.  

Infamous 2 was the inferior installment ... Simple as that.

InFamous 2 was an improvement in every way. 

Infamous 2 EU data is broken.

better availablity in store.. more stores carry infamous 1 and not 2

Kinda agree but they redemmed with festival of blood.

Idk maybe some people who did not play infamous 1 that are getting infamous second son are catching up to the franchise and story/lore. It may also be outselling it becuase infamous 2 was given away on ps+ and infamous wasn't.

Yes mechanically speaking, but the whole origin shine was on the original.  The elevation of powers from practically nothing gave me a better sense of progression than 2.  That being said I'm glad they didn't go all GoW on us and steal all the powers at the beginning because going to the next level had its own charm.  I'd say they have different charms like Batman Begins and Dark Knight.

I agree with random15.
Anyone who witnessed the awesomeness of infamous 2 from PS+ rushed out to buy infamous 1.