Nut - any surface convoy is vulnerable to land-mines, IEDs and also ambushes which may be used in conjunction with the above. The kind of kit you refer to is slow and thus vulnerable in itself.
Add in some quite long distances, the need for speed for some missions (MERT etc) plus general battlefield mobility and you can not avoid large amounts of SH and AH.
The UK's forces probably went in under-equipped with SH at the outset (not going to fire a shot, according to the then DefSec) as they were expecting to be handing out lollies and flushing loos. Since then they've been playing catch-up.