The problem is that all the action, razzmatazz, glamour and glitz of the La Liga is concentrated within the 2 giants. It has always been a duopoly.
Nobody is saying Real and Barca aren't good enough for the Prem. Real, Bayern, Barca are currently the 3 biggest clubs in the world. Nobody is denying that Real and Barca are box office.
The problem is nobody gives a fuck about most of the teams in La Liga. Even Atletico have a much smaller global fanbase than City and Spurs.
The Prem has numerous exciting teams outside the big 6 like Villa, Newcastle, Brighton, Palace, Bournemouth, Brentford, Sunderland, etc which people care and know about. Look at the signings Chelsea and Tottenham are making when they're at their worst situation in over 20 years.
Having said that, the Prem has been poor compared to its high standards over the last couple of seasons. The league is in a bit of a transition right now after the Pep/Klopp era. The only big 6 club at its best currently is Arsenal. It's still the best league though.