Because they gets pwned by voids or mass vikings...
I thought some about countering mmm push even though I never tried it
Terran: Ghost/Raven might work(EMP the medivac and pwn them with seeker missiles) or nuke.
Zerg: Baneling mines(Unless they have Raven
) Infester might be a good idea too(fungal something)
Protoss: If you're really good with mouse, block their way with Sentry's force field and pwn them with colossi. Even easier, use HT to feedback on medivac/Raven and storm the s*** out of them. DT if they forgets the Raven.