Why electromagnets over permanent magnets for a large dc machine?

The flux density of an electromagnet can be easily controlled. It's also easy and cheap to produce a large flux density with an electromagnet which is necessary for such large dc motors. Permanent magnets are generally expensive. For such a large motor,a very large permanent magnet will be needed for that motor which will not be ideal.

Electromagnets are more powerful than temporary magnets, and -as pointed out above- their flux density can be controlled.