The "artist" had been within 0.5m of live 11,000V (our safety approach is 1.2m
I used to design HV power supply units, and it's suprising how (not) far electricity will jump across a dry air barrier.
30kV DC will jump about 5mm; maybe 10mm if between sharp points. A 25kV AC wire would need to be practically touched before it will get you.
The highest I ever worked on was 500kV DC and that would jump about 1-2ft, between aluminium balls about 1ft diameter. The customer required it to withstand a number of short circuits - quite spectacular
1.2m at 11kV is hugely excessive but probably right if one is to assume the person might slip and fall over as well.
Back to the subject, 400Hz is also used to drive instrument movements that use the synchro principle; these would need to be much bigger if working at say 400Hz, and if working at much more, say 4000Hz, they would probably end up emitting noise. I believe 400Hz is also used for the fluxgate magnetometer. So there is so much stuff that has been designed around 400Hz over so many years, and I guess most of it is pretty proven and reliable, that nobody wants to change it. The electronics designers working on avionics are a long way from the sharpest - with annovation in avionics moving at snail's pace, most good people leave - and it pays to stick to tried and tested old principles.