Two main reasons:
1) It takes a lot of fire to ignite it, therefore you have to use a 209 primer to get it to go off without a hangfire/ misfire.
2) You have to use a breech plug that is designed to deliver lots of fire to the powder to light it: For example, my Omega will ignite it reliably, but my Knight KRB won't
It is funny, because in the last few years many companies redesigned the breechplug to reduce the fire so that 777 wouldn't cause the dreaded crud ring, exactly the opposite of the type of breech needed to light the blackhorn reliably