They voted it down in a referendum.

There have been a couple of problems with the EU efforts to streamline how they operate:

• There appears to be an organizational insulate many senior career bureaucrats from “political interference”, which means insulating their decisions from democracy, which was seen as a negative by the Irish voters because it is a negative.
• The voters are looking at a 250+ page document, and they have no confidence that anyone can understand what it means.

If the EU comes back with a document where decision making rests in the hands of elected representatives, and one that is short enough to understand, they will likely get approval.

The US constitution is under 10 pages. There is no reason for a governing document to be that long.