On the Twitter Help Center, the company published an article explaining the changes as well as the FAQs related to the updated policies. Twitter also mentioned in the article that by using their services after May 18th, 2015, users automatically agree to the revisions.
Ireland is widely believed to be the country that has the most relaxed privacy laws in the Europe. By shifting the rest of the world database to Ireland, Twitter ensured that it doesn’t have to comply with the NSA driven data requests every time. Facebook is another big company that went to the Irish land.
Also, as a US based company, Twitter could have found it very difficult to share data about its European users with the advertisers. Under current situation where European Regulators are showing keen interest in tracking what foreign multinationals are doing with the local European user’s data, this could have created hurdles in Twitter’s business in the future for sure.
However Twitter did not mention any of the obvious reasons for the shift, instead it said,
Reportedly Non-US accounts occupy almost 77% of the total Twitter’s user base.