Music is an universal lenguage...
You're talking about "pop music".
There're a lot of reasons why most "pop music" is mostly in english. The US market is the biggest one, english is the most spoken language in the world and every pop artist that wants to get international success is gonna sing in english. Plus, english is a great language for pop (or rock, or rap, etc) music because of its structure.
A guy like Enrique Iglesias started singing in Spanish and switched to English to try to get into the US market. Once he got there, there's no reason to get back to Spanish, as the US is market is a lot more profitable for him.
But, as you say, "Music is opinion and alot of people love them so generally speaking they are top artists. They sell loads." In your opinion, they're good. Selling a lot doesn't make you a great musician... in fact, most of the "top artists" don't even write their songs. I'm sure you can name some big stars from a few years ago that sold a lot of cds and today everybody thinks they're just pure crap
And speaking from that perspective (that of the "pop artists") there're lots of great "pop artists" out there singing in many different languages and selling lots of cds (and some of them even making good music!
). You just don't know them
The truth is, music is more than opinion, is a form of art, you can like it or not, but the "quality" of music is somehow measurable. Sadly, that doesn't apply to most "pop artists" today, they're just a marketing product in most cases with no sustance behind the looks.
This is, of course, just my opinion!