MKWii was exclusive but it still outsold most of the PS360PC biggest games. If the game is that good, people will buy the system for the game. They bought Watchdogs instead.
And if you really want to compare SKU vs SKU, just look at the OP : Watchdogs PS4 sold twice more than MK8.
MKWii's success was a combination of a good game and a very successful console. WD PS4's figures for this initial week also points to a combination of a good game and a successful console. Don't confuse the issue.
MK8 released 3 days later than WD. These figures represent 5 days of sales for WD, and 2 days of sales for MK8. Based on this fact alone, it's unsurprising that WD PS4 numbers in the US show as twice those of MK8.
These figures are only for the US, and don't even represent a full week. Trying to say at this point that WD 'beat' MK8 or that MK8 will 'beat' WD next week is meaningless.
If you really want a fairer comparison, I would suggest you look at the figures again one month, three months and six months down the line, when the difference of those 3 days becomes negligable. Since WD is cross platform, it would be fairer to look at the sales in terms of each 8th gen console's install base for a clearer indication of which game is more successful.