Marketing. The PL started earlier than everyone else and managed to establish big fan bases all around the world.
The Serie A was way ahead of the Premier League back in the mid 90s The EPL was still recovering from being banned from Europe and getting it's house in order. While Serie A was buying up the worlds best foreign talent. Even millions in England used to watch the Serie A in it's heyday when it was on mainstream TV on Saturday evenings and Sunday mornings.
The reason for the EPL's success, was because of the domestic TV rights. It's why the original big 5 (Liverpool, Man Utd, Arsenal, Everton, Tottenham) wanted to break away. Then the revenues of the English clubs skyrocketed.
People don't care about Wolves or Watford. They care about United, Liverpool, Arsenal and Chelsea. It's not about parity, it's about having more teams with a big following, and those teams being good enough to sustain interest long enough to turn it into genuine fandom.
If you said that back in 1992, when the EPL started you'd say
"People don't care about Newcastle. They care about Liverpool (18 league titles), Arsenal (10), Everton (9), and United (7)
If you said that back in 1996,
"People don't care about Chelsea. They care about United, Liverpool, Arsenal, Liverpool and Newcastle.
If you said that back in 2008,
"People don't care about Man City, they care about United, Liverpool, Arsenal, Liverpool, and Chelsea.
Anyone who is old enough to see the rise of Chelsea and Man City, would not underestimate any EPL club that has a backing. Man City went from third tier football to having a brand new stadium and winning the EPL within 10 years. Now they are selling 400,000 shirts globally every year. Yet 10 years ago they were not even near 5 figures.
The football landscape changes all the time. The only thing that seems to remain the same is that Barcelona and Real Madrid dominate La Liga.
24 clubs have won the English top division since it's inception. Only 9 La Liga clubs have won the Spanish top division since it's inception.
5 English clubs have won the European Cup since it's inception, only 2 La Liga clubs have won it.
It's the overall package that makes the EPL great.