Few reasons.
1) it never looks good in lightsaber form if it's led driven.
2) it necessitates making the hilts a little oversized to accommodate the batteries.
3) I never use light up features in my figures more than once. Get figure, open it, turn it on once, smile, then never turn it on again. It's a wasted feature.
4) it may make any figures without them look a little odd with the thinner, more accurate sabers and blades.
5) it drives up the cost unnessacarily.
For those of you that want them, great, good for you. If sideshow insists on adding this, all I ask is they at the very least continue to make and add in ones that don't light up.
Hell, even hot toys is staying away from light up sabers, due to their inability to make them not look cheap.