Yes I am bias.  
My favourite series of all time is Excalibur. The combination of Chris Claremont and Alan Davies brought me to and emerged me into the comic world. 
The original volume had great characters of Nightcrawler, Shadowcat, Rachel Summers, Captain Britain and Meggan. Three of which had already developed a great rich background and the addition of Rachel Summers opened up many questions, even Meggan was interesting.  
Yes, the title had more of a comic relief version of the X-men in the beginning but did develop into something so much better. The cast of characters was rich and deep with people like Wysdom, Kylun and Cerise coming on board. Their was so much potential but not enough readers jumped on board
The volume went for 125 issues plus annuals, one-shots and graphic novels and yet, it did stop. 
Then came Excalibur the Genosha experience.....that deserved to die. Had nothing to do with Excalibur and was just tripe. 
Along came New Excalibur with Psylocke jumping on board with Sage and  Noctunre. Hell, even Juggernaut was part of the crew. But this too succumbed to the Marvel axe.
 
Now is the reason for its discontinuation because people from the USA cant take on board a foreign team, or don't feel comfortable in a country they are not familiar with? is it because writers struggle with references not in the USA? Or is it simply fans just don't like the characters?  
Personally out of my 5 favourite comic characters, Nightcrawler, Shadowcat and Captain Britain are all from Excalibur. Having a team not based in New York, Chicago or wherever leads to the opportunity of new story arcs, new direction (say the UK makes superheroes part of the European UN forces) and just a chance for an fresh look.  
Being based in the UK also had is great advantage of the deep and rich history of England, France and the rest of Europe. Magic is of course the obvious link but the history of the area could help with stories, characters and excitement.
 
I just feel with all the cross overs of the X-titles, they could have kept Excalibur separate. Readers are able to follow one book that have great characters and an opportunity to have great, new and unique adventures that fall outside the USA Marvelverse. 
 
Guess thats my little ramble for the day.

If it were given to the right people it could acheve a Doctor Who like revival, not in terms of scale more as in people who are aquanted with the history and culture of England + Eurpe that could incorporate elements that might not have been incorporated into the books before. I don't claim to know much if anything at all about the history of The Excalibur title only that it just hasnt like you said seemed to just 'get there' yet 

I think that it just didn't get enough attention. It was a great title & one of my favorites.

Well, there's the alphabetical factor as well.  Since Excalibur doesn't actually start with 'X', it often isn't put in the same location on the shop wall as the other X books.

if by bias you mean...obviously it does fail doesnt it? so there has to be a good reason. and that reason is pretty clear.
its an extra x-book, and outshined by other x-books and extra x-books. alot of people can only afford to pull so many books.
as you mentioned citizens of the US are why this book failed...??? um, wrong! citizens of the US represented the larger part of this
books sales throughout history, as well as another European based team, X-Factor (most recent incarnation). its no more worthy
a point than saying why Big Hero 6 isnt a selling book, or Winter Guard. bottom line was Excaliber wasnt selling anywhere, or at
least enough to save it from the chopping block during downtimes.hence, since the original format was shown not to work, they tried rebooting instead. neither was Marvel UK for that matter. reason? most Europeans really only knew of the book the same way most Americans did, it was a branch off of the X-books they were reading, and would buy before it. so when one got to the store, regardless of the country they lived in and only had so much money to spare that day, it was either Uncanny, X-men, X-Force, X-Factor or Excaliber. most people arent fixated on their nationality to the point they denote their own intelligence and buy the inferior read, nor the one that isnt really closest to their heart. this is also the answer to your crossover statement. since the core readers of Excaliber were in fact X-men readers, cutting it off from those books would have caused its cancellation much sooner. US people cant write foriegn characters, pheh?
have you ever looked an an X-books team roster?

" ...Uk citizens opted to buy other books instead. "

 I didn't say US citizens were the reason failed, I didn't say US writers can't do foreign characters

Now is the reason for its discontinuation because people from the USA cant take on board a foreign team, or don't feel comfortable in a country they are not familiar with? 
 
is it because writers struggle with references not in the USA? 

With Excalibur taking place the majority in England, could some of the readers fail to understand the references made and therefor they didn't pick up or continue reading. 
 
What I said about the writers was meant to mean do they have trouble getting readers to connect to UK references

 Some of my fav writers are not US based and they write great series eg Chris Claremont, and are very successful when set in a more familiar setting. 

 They didn't mix with the others, the crisis in X-men rarely touched Excalibur and yes this does play a huge part on how successful a "spin off" team will be.  For me, I liked the fact that they were a stand alone title.

Or is it simply fans just don't like the characters? 

Think I just get disappointed when a character like Captain Britain fails to have a successful title and only has a fairly small following when they are such a rich character.