The Rugby World Cup falls during the Serie A season. Trying to use the San Siro which has games every week basically from two of the best supported clubs in Europe is very difficult. Ditto Stadio Olimpico. I think I read that Milan are trying to build a new stadium. If that is the case then that gives the organisers some flex. In 2011 Westpac Stadium could not host any other sports team for the duration of the tournament. This rule seems to have been relaxed for 2015 as it seems that several soccer stadiums will be used in England. Either way I think scheduling games could be difficult. You would need to at least get the football clubs on board to make any tournament feasible. The 2015 World Cup seems to use 5 or 6 football stadiums and only Newcastle, Aston Villa and Manchester City are playing in the top division (Leicester have no chance). An Italian World Cup could affect maybe 10 or so top division football clubs.

I'm sure you can work around it but this isn't a problem in South Africa and it isn't a problem in Ireland because the GAA seem right behind the bid.

EDIT: I just had another look at the 2015 draw.

All of the KO games are at stadiums with no football club as a tenant.

Manchester will only host one game, Newcastle will host three and Birmingham two. Leeds will host two, Leicester three, Brighton two and Milton Keynes three. Only 16 of the 48 games will be played at stadiums with a football club as a major tenant.