Paris-Roubaix is as big as Flanders in Belgium.
And is big in the département du Nord, which means Flanders, which means Belgium

(the armies of Louis XIV took it from us in unspeakable atrocities).
Milan Sanremo is big in Belgium too. Might be bigger in Belgium than it is in Italy. And as big as the other classics. I watched my first Milan-Sanremo at age 8 (1992), before I watched my first Flanders. I've allways been allowed to watch every classic, whether they're Belgian or not, BRTN or RTBF, whatever.
Also De Vlaeminck argued that the Italians have a long memory because when he once was invited in Sanremo, people recognized him.
Popularity is not just a matter of media attention. What about the people visiting the race?