In the past, armoured vehicle designers could count on most threats coming from the frontal arc.
This is why a Second World War tank like the Tiger had its thickest armour at the front, with rear armour being only half as thick, and the armour on the underside being half as thick again.
To increase protection levels, you just added more steel plate all around.
However, in modern irregular wars, the main threat may come from below, in the form of mines and IEDs.
That requires very different types of protection.
A case in point is the MR400 made by British firm Supacat, also known as the Mobility Weapon-Mounted Installation Kit, shortened to MWMIK, or more simply... The Jackal.