[Kuruharan]

Interesting question!

I'll have to do a little reading before giving you a more complete answer, but talking off the cuff I'd say that Maedhros was there first, and it became something of his own little project. He might not have welcomed "help" from Fingon.

Considering the rather rocky relationship that the two houses had in the past, it may have been more a matter of policy by Fingon to not be so involved and thus avoid antagonizing the House of Feanor. Fingon might have felt that causing that sort of split in the leadership that early might doom the cause before it begun, and the Union looked fairly hopeful (from their point of view) at the time.

And I'm not certain that Orodreth would have taken too kindly to being ordered by the High King to become involved with the House of Feanor considering his familiy's rather unpleasant past. And Fingon did not really have the means, short of a really wasteful war, to force Orodreth to do anything. (It's a chronic problem with this sort of feudalistic kingship structure.) So Fingon avoided antagonizing Orodreth.

Perhaps doing nothing seemed like the best way of keeping everyone reasonably satisfied.