Sure, engines doesn't need that energy.... If this weapon could charge from engine then why not normal versions too?
Not quite so.
recharging from engines mean:
1. the power requirements of a spinal gun is far too big for small generators to be effective.
2. spinal guns recharge rate is therefore limited by capacitor technology (with perhaps a multiplier to allow several capacitors to charge in parallel) and by total EPH. This means that if you put a big spinal gun on a ship with a low-powered engine - the recharge rate will drop. This, among other things, mean that orbital engineless guns are impossible.
3. while weapon is recharging ship will be slowed down considerably. EPH is at premium on military ships, so this requirement limits spinal guns usage to a long-range artillery and last-resort defence.
4. Spinal guns effectively use fuel as ammunition. Thim might not be a first concern of a fleet-builder, but certainly something to consider.
Therefore: normal guns are ammunition-less, more versatile in terms of ships that can mount them (low-powered patrols, stationary gun-bouys, fighters), do not slow-down the ship while recharging.