re: How come Pete Sampras never achieved the popularity of the Big 3 or Andre Agassi?

Agree with you 100%. Casual tennis fan here but watched a lot, clay court tennis is unique among most sports in that its a very small, but significant part of the sport. I would estimate of the top tennis pros, 95-97% off them are the best players on any court, including clay (I include Alcaraz and Nadal in that number). However, that last 3-5% are the clay specialist who have an outsized percentage of wins on that surface.

HOWEVER, that doesn't mean Sampras or Federer sucked on clay. Like your stat showed they were statistically better on clay than almost every other pro of their generation, but it was almost impossible to win the French or other clay matches at as high of a percentage as their career rate because they had to play that 3-5% of specialist more often, in addition to beating the other top 5%.

Not sure of any other major sport where such a small percentage of players with a very specific skill set skews the results as much as clay court tennis.