The Istari and Arnor's fall

[Inziladun]

You know, I don't recall ever considering this before.

Although we aren't told the exact arrival time of the Istari, the Tale of Years says:

c. 1100 The Wise (the Istari and the chief Eldar) discover that an evil power has made a stronghold at Dol Guldur. It is thought to be one of the Nazgûl.

Appendix A says that the Witch-king began to afflict Arnor in the time of Malvegil of Arthedain, who died in TA 1349.

So Arnor had already fragmented into the three competing mini-kingdoms by the time the Istari had arrived and were active in Middle-earth. Maybe that was a factor in their failure to aid. Perhaps they thought that any attempts to side with one kingdom or the other would only enrage the others, and any attempts to force reconciliation would not only do the same, but would be in conflict with their instructions from the Valar.

Also, though it might seem callous, the mission of the Istari was to resist Sauron, not necessarily all his minions. They already thought that the power in Dol Guldur was a Nazgûl, so they wouldn't have had any illusions that the Lord of Angmar was Sauron.