Sativa is, if I am not mistaken, the 'original ganja plant.'

Indica is a variation.

Remember....Weed has now been grown thru so many successive generations of artificial lighting.....who knows what else can change ??

Indicas for the most part, just grower shorter and more compact and flower more quickly than her older cousin.

At least that is what I think...perhaps someone more familiar with the Indica strain can answer.

I think the way you ask...."why do sativa take so much longer?"

could also be asked...."why do indica flower so much earlier?"