There was a malfunction according to most reports. There are many possible failures where the jet could fly but ejection was still the best choice. For example

There was a malfunction according to most reports. There are many possible failures where the jet could fly but ejection was still the best choice. For example total loss of fight instruments in IMC (no / low visibility). It could then fly straight if you're lucky, but given that you have no way to crosscheck and may be about to crash, ejection is the right choice.