I don't think there is a scientific or a spiritual reason for 5 kakars.
Lets be honest, Hairs as receiver for energy doesn't really make much sense.
Personally I believe 5 kakars was a military directive from Guru ji and that's why we don't find detailed explanation for each kakar as military directives are usually '
Precise' but binding.
The reasons according my own very personal opinion -
Hairs - To give a fierce, intimidating look to Khalsa.
Comb - To clean hairs.
Kirpan - Always be battle ready.
Kara - To tell own soldiers from the enemy and identify 'Shaheed' soldiers.
Kachera - Would be difficult to fight with one hand with other holding dhoti
So kachera is better.
Guru Gobind Singh was a brilliant military general, with 5 K s he gave an effective dress code to Khalsa and with '
Amrit' he took the fear of death out of them.
Thus turning sheep into Lion.