There are two fundamental problems with flying in volcanic ash:
a) We don't have a good method of determining where it is and at what concentration, and
b) We don't have a good understanding of at what concentration it becomes hazardous to aircraft.
Bird strikes can be (and have been) a problem - so there are standards and regulations. Manufactures need to demonstrate that their aircraft/engines can deal with a defined bird strike threat before they can be certified.
No one has ever done a test on a high bypass jet engine to determine at what concentration ash becomes a problem. We know the ~1gram/cubic meter that the KLM 747 experienced was a big problem. But what about 1 milligram/meter, or 1 microgram/meter? And how sensitive are different engines - would a concentration that was OK for a 1970s JT9D be OK for 21st century GEnx (or visa-versa)?
No one knows, and no one is willing to fund the necessary investigations and developments (at least so far). So for the time being, the only viable answer is complete avoidance.