Seen some interesting speculation from a US-based scientist and research analyst:
H5N1 is relatively unstable in water - lasting only 2-3 days in water at 17-28C - in contrast to natural H5/H7, which can survive for months in water (even years in cold water?).
At the same time, it is apparently better at surviving in bird faeces. (Research by Webster's team at St Jude's)
Taken together, these findings may indicate the virus is better adapted to survival in poultry farms than in the natural environment (ie in wetlands, a key habitat for wild bird flus).