Wimbledon should not be considered the most prestigious major—so long as you tie prestige to competition. Grass courts are inaccessible to the majority of youth tennis players. This means that grass surfaces are the least practiced along with clay. At the same time, the fact that a select few youth players do have access to clay courts breeds inequality of opportunity and affords them a leg up over their peers.
On the other hand, the vast majority of players have access to a hard court. It is likely that any given player’s earliest exposure to tennis occurred on a hard court. And for players like Serena and Venus, a hard court was all they knew until the day they turned pro.
As a result, hard courts are the most practiced courts for these players and where they are able to play their highest level of tennis. I would even bet that in absolute terms, Nadal is a better player on hard courts than clay, even if in relative terms the gap between him and his peers is larger on clay.
With that said, the highest levels of tennis are achieved on hard courts—which should therefore be considered the most prestigious majors.
Well, it just doesn’t work that way. Tennis was invented in the UK, first played on grass, and Wimbledon is the oldest tennis tournament in the world, major or not, and gets attended by the royals, lots of celebrities etc and maintains the most traditions, for better or for worse, so it just has a certain air of prestige that the others don’t.
Doesn’t mean the best quality of tennis gets played there, but most kids grow up wanting to win Wimbledon, even Nadal wanted to win it more than the French as a kid.
Technically tennis was invented in France, but UK has a lot to do with popularising it.