> It seems a little strange that more advanced later machines like the original PDP-11 from 1970, couldn't "autostart" from ROM like that.

Because the PDPs had magnetic core memory, so the bootloader, once toggled in, stayed in memory. The OS took care not to overwrite it.

And in normal operation you wouldn't have to use it at all, you just switched on your PDP, and it was in the same state as when you switched it off.

Also, bootloaders were short (because peripherals had lots of intelligence); toggling one in was a matter of minutes.

> I think that was because core memory was non-volatile.

Exactly. And when RAM cards were available to replace core (very late in the lifecycle of the minis), the RAM was battery backed to simulate that behaviour.