IRUs are expensive.About $250,000 each. Airplanes capable of oceanic or remote land mass flights almost always have three. The cost is justified for that purpose.
RNP AR requires two sources of position information for RNP of less than 0.30 or missed approach of less than 1.0. (also known as dual string)
The three IRU airplanes are good to go simply because of their oceanic/remote land mass nav capability.
The FAA, at least on paper, will accept one IRU for dual string. But, $250,000 is a stiff equippage price for a domestic aircraft to simply satisfy the entry into the RNP AR "dual string club."
An IRU with at least the accuracy of the Honeywell unit for a fraction of the price would likely find a lot of buyers. It wouldn't have to provide an attitude platform, just provide good position information for a maximum of 30 minutes in the event of loss of GPS during a dual-string RNP AR IAP.