It's time & effort.
The Negatives go through a minilab, pretty much automatically nad usually very cheap. The slide processing may be using a manual Dip & Dunk machine or small automated system, but through put is much lower overheads higher, tighter quality control, far more chemistry, semi manual mounting etc, so you pay more.
I'd add my lab used to run E6 continuously, they were extremely busy, last time I ent it was down to 3 process runs a week, they closed early this year. E6 isn't economic any more, a UK city which had maybe 15-16 E6 labs now has 1.
Ian