There are multiple factors ...
1) Price
While being inexpensive may not help sales, being expensive certainly hurts sales.
2) If your friends and family has the system
You're far more likely to buy a system you have first hand experience with, and you are much more likely to have first hand experience with a system someone you know owns. Beyond that, you are probably going to hear about games they're playing on their systems, be encouraged to play with them online, and (partially) be motivated by the potential of sharing games.
3) New/innovative/different experiences
People are very likely to buy new hardware if it offers something substantially different from hardware they already own
4) Familiar franchises
While people do want new experiences they're also driven to play "better" versions of games they already liked
There are lots of other factors which have a minor influence, but I believe I've captured the major ones. The ones I've listed also (indirectly) explain why the first mover has an advantage in the market ... 1) At any point in time they're likely going to be less expensive than their competition, 2) When the trailing system's launch there will be significantly more people who's family and friends own the system, 3) Since they release earlier most improvements over the previous generation will be initially attributed to them, 4) By the time the competition releases the system will be seeing many games in well known franchises







