Its based on UEFA coefficient, dont know if it was calculated the same way as now though:
https://en.m.wikipedia.org/wiki/UEFA_coefficient
None other league has 3 clubs historically as great as Juve, Milan and inter. None other league has had as many different UEFA competitions winners as Italy (11, Spain has 9). Spain has the most trophies (55, Italy 48), but thats mostly thanks to their recent success of Sevilla, Barca and Madrid (they won like 4 trophies a season for 3 years). Serie A has also had the most different Balon dor winners (15, Spain had 10), and tied for total wins (again thanks to their recent domination with Messi and Ronaldo, and only because were counting Cannavaro 06 and Ronaldo 02 for both). If it wasnt for their recent success this wouldnt even be a discussion, but thanks to them winning so much on the last 10 years they closed the gap to Serie A
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Forgot one thing to mention, number of trophies doesent have to mean much. If you have 3 english clubs in the semifinal and Barcelona wins (thanks to refs, got destroyed vs Chelsea), does than mean La Liga is the strongest league for that year?