Why should the wire of the loop of a solenoid be copper and not made of iron?

Answer 1

Because it's a better solenoid if its copper, due to it being a better conductor than iron. It thus takes less voltage to produce the required magnetic field, and also this leads to less heating-up of the coil.

But, -of course, you could make a perfectly good solenoid with iron wire if you wished. It would be a bit "iffy" because the iron coils would react with the magnetic fields produced, and might hum or rattle -!!
But if (1) this didn't matter, and (2) the heating effect wasn't a problem, and (3) you've got voltage enough and to spare, and (4) you've got lots of iron wire but no copper wire, then there's nothing to stop you making a solenoid out of iron wire!! - or carbon, or lead, silver, pewter, -whatever.

It's the magnetic field produced by current passing through a conductor of any sort, that's harnessed to use as a solenoid.

Answer 2

Iron wire wouldn't bend very well and wouldn't conduct current as well.

Answer 3

because iron is magnetic.