Iron cylinders don't need to be plated. The iron sleeve takes the place of the plating.
Basically, the cylinder cannot be aluminum because the aluminum piston and cylinder would rub against one another, and aluminum being soft it would seize up within short order. It has to do with the hardness and coefficient of friction. Iron/steel has a very low coefficient of friction and is much harder than aluminum, so when other metals rub against it they tend to just slide across one another - this is why trains are so efficient they ride steel wheels on steel rails. The reason Chrome, Nickel Silicon Carbide, and ceramic coatings are used in place of iron is for the same reason. The materials are relatively hard compared to aluminum and have extremely low coefficient of frictions - even better than iron.
The big advantage to an iron sleeve comes down to reliability. Cast iron is very porous, so it does an extremely good job at retaining oil which has benefits for longevity. This is why iron engine blocks, and even iron sleeved aluminum engines, were used in car engines.
The other benefit of iron cylinders is that the liner can often be lightly honed and re-used during a rebuild, so as long as no major damage has been done, and it hasn't been worn too far out of round, the cylinder can be re-used during a rebuild.
There are technically downsides to iron sleeves. Iron doesn't conduct heat as well as aluminum, so they do suffer a bit in terms of thermal efficiency. You also need to avoid pushing them hard from a cold start because the iron expands slower than the aluminum around it, so for the first couple of minutes you want to take it easy and not push very hard, otherwise the sleeve can shift/move. It's not common, but a possibility. Another downside is the iron sleeved cylinders are a bit heavier. Not a ton of weight, but its still technically a "negative" aspect.
Looking at the platings, they too have their upsides and downsides. One big downside to all of the platings is that they are plated. All types of platings used are very hard substances, but also relatively brittle, so when the plating fails it starts to chip and flake off, and once they flake off the alumimum is exposed and failure starts to happen. Iron sleeves don't chip and flake.
Platings are also very thin, so the transfer of thermal energy is much higher, aka better at shedding heat.
The big advantage of all of the plating materials mentioned goes back to their very low coefficients of friction. Especially Chromium oxide, which is the best of all in that regard.
The newer ceramic coatings like the one used on the Phantom 85 are not too far behind Chromium in terms of coefficient of friction, but also have other benefits, which is why they are starting to become a new standard. Ceramic lined cylinders, like iron, are more porous, so they do a good job of retaining oil. They also resist absorbing heat, which improves the thermal efficiency of the engines. And on top of all of that, they still have a lower coefficient of friction than iron.
Nikasil is a compromise coating. It's not quite as good in regards to its coefficient of friction as chromium, but has much better durability and some of the thermal efficiency benefits of ceramics since the base substrate is technically a ceramic. It's also more expensive, which is why only the higher end cylinders tend to use Nikasil, and the more budget friendly use chromium.
I could keep going, but this is probably enough materials science for now.
