Why we have internal oil wetted flame trap and not external or dry types??

Why we have internal oil wetted flame trap and not external or dry types??

If flame-trap is placed on the outside of the relief valve, the flame is concentrated on a small area by the directive effect of the valve aperture when open, with subsequent destruction of the
flame-trap.

The internal flame-trap avoids this effect due to the more even distribution of flow and so is capable of absorbing approximately three times as much heat.
It has been further demonstrated that the capacity of the internal flame-trap gauzes is roughly doubled by coating them with lubricating oil, as can be done by arranging
them in the path of oil spray or splash from the large end bearings of the connecting rods or supplying oil to them by alternative methods.
The combination of these two. features makes the internal, oil wetted, flame-trap about six times as effective as an external gauze flame-trap