Helmet fire, the electric start is a big pain in the arse on the ST ,2xbloody heavy batteries at approx 50 kg ea ,a large elec starter on each eng, which is only a starter and the elec nightmare with 40 + relays to start, you only get three attempts to start an ST ,if you stuff it up then your stuck as the batteries give 40 volt during start and ext power is only 28 so you are just about guaranteed a long hot start on the first eng!!
APU's are far suppiror, you have full AC power ,Hyd power, and below 25oC you can do a dual engine start , and the whole APU system only weighs slightly more in a Blackhawk than the elec start system in a ST, it also allows your Helo to be self sufficient , with minimal GSE.