The volcanic ash is traveling in high altitudes exactly where the jet streams are, which are used by the airplanes.

A few hundreds of miles off the vulcan the ash is already thinly distributed. Looking up from the ground the sky may still appear blue.
What makes the ash so dangerous to aircrafts is three things:
1) the ash consists of silica particles. If they enter the turbines they will solidify to glass-like substances, choking the engine
2) being silica particles and considering the high speed of airplanes the ash acts like sand paper, eroding surfaces on the wings of the airplane which consequently suffer in their aerodynamics
3) worst of all: the ash cloud is not visible to any onboard radar technologies ...
... so the pilot will realize the ash cloud too late