In Test flight, a spin chute can be fitted. That is absolutely not necessary in a transport category Aircraft. All a/c can stall, and all systems are subject to failure.
447 was lost due influences absolutely designed for. Whether a Training syllabus blindspot, or a glitch in elec/mech, it happened, and BEA know how.
"Why" is different, notably, from how. It will take a judge to determine (jury?).
Why involves the human factor, and as such, we consequently live in a fail/possible realm...
And even then, not all will be satisfied.