It's a way for them to exert influence. For example, a Turkish party (DENK), considered by some to be a front for Erdoğan's party, won seats in the Dutch parliament in the last elections.
Those 10% are people living in Istanbul, which is partly in Europe. Nationals of other countries, but with Turkish roots, usually aren't counted.
Not that Turkish nationalists in European countries is something we're all happy about...
I'm not sure if legally they are still a candidate for the EU but for practical purposes that initiative is dead.
Legality and legality...
Turkey is a candidate, because they chose to apply. But to fully qualify for membership, they need to adhere to the Copenhagen Criteria:
- stable institutions guaranteeing democracy, the rule of law, human rights and respect for and protection of minorities;
- a functioning market economy and the capacity to cope with competition and market forces in the EU;
- the ability to take on and implement effectively the obligations of membership, including adherence to the aims of political, economic and monetary union.
More specifically, the Copenhagen Criteria rules are divided into
35 different policy fields (often just called chapters). There can also be specific country-based rules which are discussed during negotiations. IIRC, Turkey started work on aligning itself with just two of the chapters, and has yet to fulfill a single one!
In practice, Turkey is and was never a candidate for membership. Not because it wasn't allowed, but because
Turkey does not want to abide by the rules for membership.
Whenever they change their mind, and change their country to fulfill all the criteria, they'll be much more likely to be accepted. It is generally acknowledged that the EU expanded a bit to fast in Eastern Europe however, so there might very well be much stricter guidelines before finally accepting Turkey -- or any other state -- as a new member.