When a main opponent goes, you go with him, look at Van Der Poel yesterday on Taaienberg. Or if you know 100% that your team is capable of pulling it back, which was certainly not the case when his strongest teammate was on the attack for 50km, plus it is a team which rides together once a year, so you never know for sure.
It is true that Belgium should've ridden differently, primarily in Wout's service, but it is aslo true that Alaphilippe was hands down strongest rider of the day, and I'm not sure how the outcome would be any different.