A headlamp might have not cost very much in terms of parts, but the additional labour which would have gone into assembling each & every cleaner with a headlamp would have been considerable. On UK cleaners I think the Hoover Turbopower was just about the only one where additional assembly time was kept to a minimum, but even then it would have added up.
Clearly there were two camps - those manufacturers who saw a headlamp as a selling point, and those who didn't. Electrolux never had headlamps on uprights, nor did they have automatic cordwinders on any of their uprights, save for one model of the Glider in the late 80's / early 90's which was very short lived. Electrolux always sold themselves on the performance and quality of their cleaners, rather than adding on subjective extras which the competition were using.
You only have to look at the Moulinex Major to find a cleaner which had everything on it and still cost less than many of the others. Popular as it was, it doesn't appear to have impacted the sales of the less-fancy Electrolux 500 cleaners.