I know this might seem like an odd question, but is there any reason that Sun and Moon get released in Europe several days later than the rest of the world? This happened with OR/AS as well.
It's all about shipping issues such as rates, licensing and finding all the different places to retail it.
Hope I helped!
There is no clear reason ever given by Nintendo/TPCi. Logistics is a possibility like Artist KS said, but they managed it fine with X/Y so it seems unlikely.
The most likely reason seems to be the "leaking" issues that happened with X/Y. Several shops in Europe (mostly Italy and Spain I think) broke the street date so some people got the game a week in advance and leaked tons of details before anyone else got the game. (This did also happen in Canada but the shop that did it got fined heavily and had to close.)
Having said that, they are not delaying the release in China which is notorious for piracy. Hopefully they don't end up regretting that...