That's not specific to this build, that's general advice.
In end-game content in Greater Rifts, these are the most dangerous damage types.
This includes the Khazra Spear-throwers, whose projectiles may be hard to spot in the chaos of all the special effects, while still taking out a huge chunk of your health pool. A more fragile build attempting to push GR can get 1-shot by these.
It's also just the most common damage type outside elites. If you're grouping some 100-ish monsters together, Physical is probably the most used one, and you're probably going to take a few hits here and there simply because you can't keep track of every single monster's attack animation.
Lightning Resistance is primarily to defend against the Electrified elite Affix if you're melee - otherwise you might need to skip these elites.
Thunderstorm can also easily kill you if it hits multiple times, and Lightning Resistance helps to blunt the impact somewhat.
Why not the others
Other affixes tend to be dodge-able or to have lower damage. For example, Molten death explosion will do far more damage than Thunderstorm, but it's easy to dodge. Frozen Pulse is nasty, but can be avoided reliably with only a single tick. Very mobile builds sometimes even without taking hits.