Definitely (1), but that is the same as a lot of other great MTB destinations in Italy
2) I think a lot of it has to do with people liking to drive to MTB holidays, all the spares, toolkit etc. with sardinina this is easy for italians who take the ferry.
3) Definitely, I am sure you are used it now but for the average brit June/July and August in more southern parts of Italy can be too hot for sporting activity (destinations in Liguria etc can have the same problem)
4) If I remember correctly the race you mention is similar to a road sportive but off road and over a couple of days, IMO the average uk mountainbiker does not enjoy these hard events, rather riding laps round a field and drinking beer. The italians / belgians / spanish are much more into the XC enduro side whereas in the UK the sport is dominated by middle-aged all-mountain dudes more focused in railing berms and fettling in the garage than fitness..
To qualify my comments I have been to Alghero on holiday and am regularly in Cagliari on business (visiting Tiscali), I lived near lake Como for 3 years and have ridden in lots of areas around italy (mainly northern) – my wife and daughter are both italian.
I love Sardinia but would always take my road bike due to the challenge of finding off road routes. Porchetta, pecorino sardo, mirto…..mmmmmmmmmm