Why? I have thought it over. I don't see how we can argue that van Looy was better than De Vlaeminck was.
The latter was much lighter and hence could climb. He was more much more complete. OK as Van Looy was so heavy, you can argue it was a big deal to see him climb so well at times.
It's often been said that Van Looy had a better reputation because he was the best rider of his era (in one day races at least) and because he was more of an attacker, while De Vlaeminck so often drafted Merckx's or Maertens' wheel.
But in my view, building his palmares under Merckx makes De Vlaeminck even greater. That surely was the greatest era for cycling. And Van Looy also rode very negatively against Merckx (RvV '68) while De Vlaeminck had beautiful solo rides that Van Looy never had (Emilia '76 >80km solo, Roubaix '77> 35km solo).
The De Vlaeminck palmares seems clearly better as such to me and the way he won them also were better. Van Looy won some of his best races in mass sprints, just like a Pre-Poggio Milan Sanremo and Lombardy when the finish was still in Milan's Vigorelli and his Worlds. No way Van Looy could have won Lombardy on a present-day route. De Vlaeminck won both Lombardy's in Como with huge climbs and his 3 Milan Sanremo in breakaways, the largest group consisting of 15 riders.
And don't forget that De Vlaeminck was one of the greatest cross specialists of his era. Van Looy was a track specialist but De Vlaeminck could do well on the track. And he also won his share of Italian one-day races that he loved so much(he considered them all as Ardennes classic equivalents) while in Italy Van Looy won sprint races like Sassari-Cagliari, and that's about it.
OK That was my opinion.
I think I gave my top10 earlier on this thread and some contested it.