The main reason is that unlike DOT fluid which is hydroscopic (it absorbs water from the atmosphere, thus lowering the boiling point of the fluid) Mineral oil is hydrophobic (it does NOT absorb water from the atmosphere so it's boiling point remains constant).
Of course, this is only true if water can never enter the clutch/brake system. If water does enter the system on a K clutch, then the water is not absorbed by the fluid but rather sinks to the lowest point of the system which is normally the clutch slave cylinder (as in our case) which also happens to be the hottest part of the system.
So, if this water gets hot and boils, the air given off reduces the effectiveness of the clutch hydraulic pressure system. It might also mean that the water will boil out of the system...
It also means that it is easy to remove any water by a partial bleed of the system, so removing any fluid at the lowest point.