Mostly 2.4GHz is used purely on a safety basis. With robots like these safety is a big issue so radios that won't be interfered with and are fairly standard are high on the list. I know you have concerns over putting a 2.4GHz system in a metal armoured robot but it never really comes up as an issue even in robots that are just pure metal boxes! No idea how a UHF system would play in the setting but going with tried and tested is a good idea.