It's not that "suddenly everybody wants it" - there literally haven't been any changes to Croatia (or Serbia) policy regarding the border. However, they are im

It's not that "suddenly everybody wants it" - there literally haven't been any changes to Croatia (or Serbia) policy regarding the border. However, they are implicitly reasserting that no, Liberland did not establish a country there, that Croatia doesn't consider this as a state, that there is no such thing as Liberland. And perhaps they are intentionally making a point of it to demonstrate that yes, they can and will push it out if you make unwarranted claims of sovereignty without their consent - for example, prosecuting individuals for using fictitious passports, as according to Croatia Liberland and its passports are fictitious. The key part of setting up a new state is an agreement with your neighbours that they acknowledge your sovereignty over certain borders (i.e. "play along with their political games"), which certainly is possible as there have been multiple new states set up during 21st century, but until that is achieved, you don't really have a state.