The Tour players started using it 2-3 years ago, in the earlier versions, and they are now used to the feel of it. They get it for free and don't use the same string for more than 2 hours max. After that it goes dead and plays like a board.
Newer and more advanced CoPoly started coming and right now the Luxilon strings are old technology, but pro players don't like to change racquets nor string brand very often, so Luxilon is still the most used string on tour bar none.
Nadal also still plays with a very boardy stiff and plain Babolat poly string, because it's what he is used to play with. The players only change string brand, if their game goes south.
Luxilon lives high on that reputation - and sell a lot of strings because of this. I love Luxilon Alu BB ROUGH the first 2-3 hours, after that I get Tennis Elbow from it and need to cut it out.
I find that there are better performing copoly strings out there now, with far better tension stability - so I dumped Luxilon last month and will not but it again until they make a proper 4.th generation string with focus on better tension retention.
Weiscannon, Signum, Tecnifibre (French company) and other German companies now make better CoPoly strings than Luxilon, I hate to say it, but it's true.
Some still WANT AND NEED to believe LUX ALU BB Rough is the best because Fed, Djokster and a lot of other pro players use it. But as I say - even if there are better racquets out there, a pro player does not change his racquet unless something is wrong with his game. Same with strings.