Why have none of the US /UK/Japanese airlines ordered the Irkut MC-21 yet?
Ordering an aircraft is a very complex business, and implicates a number of matters that don't actually (on the surface, anyway) have anything to do with the aircraft itself. Consider this from the point of view of a western European or north American airline. 

Broadly political obstacles

These could include:


international trade agreements and disputes (Russia's relations with much of the world are not very friendly at present)
existing suppliers ("Lovely relationship we've enjoyed since the 1970s; it would be a shame if someone carelessly trampled it underfoot and soured it.")
governments and politicians ("Oh really? Well, it certainly has been nice supporting you fearlessly in Parliament/the Ministry/Congress/against my opponents who hate your guts. Bye!")
national expectation ("Traitors!")


History and reputation

Russian (like Chinese) design, technology, engineering and manufacturing have a reputation in the west that works against them. These things take decades, not years, to be overcome by stellar achievements in those fields (consider how long it took "Japanese" to become regarded as a positive rather than a negative adjective when applied to products).

However unfairly, this feeds in to other obstacles:


customers ("Get on a Russian airliner? No way!") 
shareholders ("You're proposing to buy a what?!") 


Risks

Then, it doesn't make sense to buy one plane to "try it out". Even with a single aircraft of the type, you'd need the same training, certification, engineering support, etc etc as if you had a fleet of them.

So, you'd be looking at a fleet of them, and now you're facing the purchase of a fleet of aircraft that very much represent an unknown quantity. It's a huge, huge risk; no wonder airlines cleave so strongly to the Airbus/Boeing duopoly - maybe it's not entirely to their economic advantage, but they don't expect too many surprises, which is arguably more important.

Some of these risks: 


are simply technical ("How good will the product turn out to be in the long run?")
are economic ("Could the trade climate make spare parts and maintenance prohibitively expensive in the future?")
hinge on international relations ("Could our government ban us from working with this supplier in future? Could a meddling Kremlin disadvantage us?").


How could it work? Will it ever work?

Give it a couple of decades. 

If airlines in places like India, Indonesia, Brazil, with fewer historical attachments and political obstacles than in the west, find that the aircraft and manufacturer prove to be good for them, bit by bit we may see that influence trickle into western airlines, especially as those non-western airlines and the economies they belong to expand and become more influential themselves.