Because if you have any non emergency lights wired in with the maintained fitting they will still be operational when testing the emergency lights. Your way will work, but will kill all lights connected from that switch, not just the EM lights. There is nothing wrong regulatory speaking with doing it your way, however if testing is to be done at times whereby the switching off of luminaires becomes an inconvenience to the users of the installation then BS 5266 would call for it to be done my way, habitually this is the way I always do it. See below:
My way, the normal lights stay operational when key switch isolates the permanent line and EM light:
View attachment 17137
Your way, all lights go off when key switch isolates the permanent line and EM light:
View attachment 17138