Question says it all. Most teams struggle when it comes to away and by that I mean against all teams barring the minnows. Then again, the better teams have now started to struggle in places like Zimbabwe and Bangladesh so another question could be "Why touring teams struggle to also compete"?
Recent examples are i.e. Australia thrashing India in their grounds but vice versa away. West Indies thrashing New Zealand at home in 2012 but vice versa this year.
Even Ireland are tough to play against at their grounds but they are currently touring the Caribbean and getting thrashed by teams like Guyana and Jamaica despite the fact that these teams are just small parts of the West Indies.