There are many things that we still do not know: I've seen nothing about fire alarms, for instance.

But the fire was clearly not contained in the apartment of fire origin (apparently the fridge was near the window) and the fire spread quickly outside the building. Europe has been very warm and many residents may have left their windows open. Or the heat may have broken windows as the fire ascended.

Also, there are reports that the Celotex product contains polyisocyanurate which can produce hydrogen cyanide gas among other things, none of which would be helpful to residents trying to escape (and especially problematic for those who had decided to "shelter in place").