Vlaanderen, on the new route, has lost a large amount of it's tactical possibilities and now the strongest guys just duke it out 100% of the time. Same with Liege and Il Lombardia.
The different nature of Roubaix and Sanremo gives two vastly different profiles of surprise winners as well. In Sanremo the traits that make a surprise winner tend to still be explosive riders, good finishers and puncheurs, so you still get reasonably prolific winners.
Roubaix on the other hand, is such a race of attrition, which in turn is such a rare race that lends itself for riders to excel there and nowhere else. In a similar way, Liege and Lombardia don't really get dank winners cause even when they have a surprise winner, it's a well known climber/GC rider with a string of results i.e. Mollema or Pinot.