[Dave Moll]

By 1984 all the 405-line transmissions were duplicates of what was available on 625 lines, so I imagine the main factors were getting rid of all the additional equipment (standards converters and VHF transmitters), and freeing up the VHF spectrum so that it could be sold off to other potential users. Not to mention allowing manufacturers to cease production of dual-standard televisions.

I seem to recall that the final switch-off in 1984 was somewhat later than intended because of the number of viewers with 405-only televisions still in use.

I think the demise of 405 lines was set in motion with the decision to make BBC2 a 625-line only service