Because the power on the good engine somehow decreases significantly. Yet the Algerian investigation board doesn't make any attempt to explain this very crucial event.
Very interesting indeed. The report (which is now crashing acrobat for me, so excuse me if I get a few details slightly off) has a bunch of findings and conclusions, which are not very complementary to the way the crew managed the flight:
The only attempt at a checklist was interrupted when the Chief cabin crew member came into the cockpit and talked with the captain during taxi.
No briefing was made.
After the engine failure, the captain took over control, without ascertaining the problem.
The gear were never retracted
The captain tried to maintain an 18-degree pitch -- not ideal for post engine failure -- and the aircraft stalled and crashed.
yet, the report also has:
A) In the simulator, even maintaining 18-degree nose-up pitch and gear down, the aircraft would continue to fly -- a reduction in power on the good (right) engine was needed for the aircraft to crash.
B) Boeing's analysis of the FDR includes the statement that only a drop in (right) engine power could explain the plots.
C) the left engine (which failed) Fuel Flow gauge was recovered reading something like 4800 kg/hour. The right engine FF was at 200 kg/hour.
(before you ask, the throttles were recovered free-moving)