From the description of the attack it seems like the only reason other machines aren't affected is because the Russian group running this hasn't gotten their hands on those machines.
A hardware RNG is actually pretty cheap to make, there are actually open source (as in the circuit diagrams are available) ones floating around the internet that you can build for less than $20 worth of parts. The manufacturers probably don't want to go that far because a) compared to most of the parts in the machine it's a fairly expensive piece so it will cut into profits, b) hardware RNG function a bit different from a PRNG so properly integrating one requires a certain amount of skill, c) it isn't their problem really, they already got their money, it's the casinos that are losing, and d) it's simpler to just prosecute the handful of people doing this (for now).