Electric starter and a move towards alternators instead of generators, which helped make the electrical system more reliable, so no more need for it.
Many makers started providing an installable kick lever under the seat to show how reliable they were, bmw for one, then as Kslrr says, somewhere in the late 70's, early 80's they realized how much money they could save by not installing one.
I prefer a kickstart, no real reason why, just prefer having them, seems a bike does'nt look right without one.
Ken.