The move into the web alone was responsible for a lot of that killing. Up to this day web tools are not as good as the old desktop ones, but those old ones are all dying. Mobile devices also killed some stuff, but much less.

Before that, computer networking killed most of the running systems. And before those, any new computer would require all new software, so there was no continuation at all.

Things seem to be getting slower with time... maybe except on Javascript frameworks.