Sorry, at least one M1 has done so. Like the Merkavas, it didn't survive, but half the crew survived - which you can't say about the Merks - one Mk 2 Merk was completely destroyed with the loss of the entire crew when it rolled over a Hez IED, estimated at only 50kg. There was at least one other Merk destroyed by a Hez IED, with no reports of survivors (but no confirmed deaths, either). So, the Merk is by no means immune to IEDs.
Merk2 production stopped about 1989, it is an honest, but obsolete living machine. I'd want to see what a 50kg charge might do to an Abrams.
So, the Merk is by no means immune to IEDs.
What is? It's a question of pounds.