Why are sash windows best for Fire Egress, well mainly because most sash windows are large, the clear opening size of the lower sash provides excellent void to escape through, should the need arise.
Unfortunately, non professional window companies, have over the years fitted the cheapest solution to ensure they win the work, but being the lowest price comes at a cost.
Many window installers fitted thin, aluminium framed “inserts” into existing wooden box frames (because it is very quick, cheap and easy) these generally consisted of one large, fully fixed pane of glazing with a narrow fan light above. Fixed panes of glazing prevent easy exit, requiring the occupier to smash the toughened glass or try to climb above the fixed portion of glazing, making it virtually impossible to escape.
Sash windows have always cost more than casement styled windows, there is more material, more labour and more skill required, however the finished installation looks infinitely better, provides significant ventilation and excellent exit should the need arise.