Because strength of opposition matters, and Usman, with the exception of Woodley and Covington, has not faced elite opposition.
These guys and Masvidal are his best wins by far, whereas St-Pierre has numerous great wins.
That aside, St-Pierre trumps Usman in pretty much every, if not every statistic when it comes to MMA and in most cases, it's not even close, as i've shown you.
Fighting is first and foremost, about winning and both Usman and St-Pierre are extremely good at that.
St-Pierre is however not as much of a "safe fighter" as Usman is:
View attachment 811864
Cheers!