Is there a functional reason for Ghostfire not having the Devoid keyword?
Yes, there is a practical difference between Ghostfire and Reality Hemorrhage as far as the color-setting ability goes.

The colored mana produced by Corrupted Crossroads could not be used to cast Ghostfire.

Other examples of cards affecting other cards with specific keyword abilities include at least the cards found by this search, except Humility. Note that this was a very basic search, and there are many more such cards that affect specific keyword abilities, such as Adarkar Windform.

It seems unlikely that Ghostfire, a card released in 2007 as part of the out-of-order Future Sight edition and the only card made colorless by a CDA before the Battle for Zendikar block, was intentionally left un-errata'd to nerf its interaction with Corrupted Crossroads. Avoiding unnecessary erratas seems to be the most likely explanation why it was not updated.