1. 2% is not a requirment, but a recommendation. That is why their are no repercussions for countries spending less.
2.Economic Differences. A richer country that spends less then 2% may still end up paying for more then a poorer country that spends a greater amount of their GDP.
3. Forcing Countries to spend a fixed amount of GDP is wasteful. Great they spend 2% of their GDP, now the Military has uneeded Vehicles and parts sitting away in a warhouse. Good news tough is that some Technical Museum in 50 years will have mint condition exhibits.
4. and parts of the EU are disillusioned with the US. They dragged NATO into a non defensive War (and some may argue non necessary war).
5. Reallying on the US is a bad idea. They have shown they will withdraw and abandon allies.