Despite the outcry from some posters above in the light of the Boeing bulletin and FAA directive, the fact remains that existing procedures should have brought this aircraft safely back to ground.
The bulletin and the AD both say this.
It appears that maintenance and training are the major contributors to this tragic accident.
Maintenance: a maintenance culture that allows an aircraft to fly 4 successive sectors with a known issue affecting air data is completely unacceptable, and it certainly wouldn’t happen in any airline with a proper attitude towards maintenance and safety.
Training: given the above was allowed to occur, a well trained crew should still have been able to handle it. In the absence of well trained crew, an accident was simply a matter of time.
There is no such thing as pilot-error in an airline, only a poor training system which allows poorly trained pilots to operate.