Why?

Because the issue is not about joining the EU. Ukraine has not applied for EU membership, is not a candidate country for EU membership, and there's no reason for having a refenrendum about membership.

Ukraine is one of six EU Eastern Partnership countries with Armenia, Azerbaijan, Belarus, Georgia, and Moldova. Eastern Partnership is a EU foreign policy framework of policies and bilateral treaties to facilitate co-operation with those countries, and to promote democracy and rule of law, to support economic and social development, to reduce social inequality by funding, to promote enviromental protection, to encourge personal contacts and travel by forming visa free regimes etc.

What originally sparked the current protests in Ukraine was their president's seemingly sudden about turn not to sign a long negotiated Association Agreement with the EU. The AA creates a framework for further co-operation for example by giving a free access to the EU's single market while including commitments to democratic, judicial, economic, human rights etc. reforms.

The AA is not a EU membership application. And while the closer co-operation and reforms may bring the country closer to fulfill the Copenhagen Criteria for a possible future membership the AA does not promise such.

And did it also happen to other countries that they joined the EU without the population voting on it?

Before accession to EU membership a country has to fulfill the Copenhagen Criteria. Having a refenrendum is not mandatory as approval of membership can also be expressed by national parliament. Yet most of the current 28 EU members have had a referendum about the EU accession.