It is a very human trait not to want to admit your mistakes - especially not when there might be consequences.
Part of the ease with which Germany excused itself for the war crimes in World War II can be explained by the fact that there wouldn't be consequences: the West needed Germany much too much as an ally in the Cold War. It didn't make up with the countries to its east until much later but nobody expected that in the heat of the Cold War.
Japan on the other hand had a more vulnerable position. In its region the Cold War was much less important. And many countries demanded excuses and indemnifications. Taking a humble approach would only result in more demands. So Japan had to adopt a rather strict approach. Sure, they might have been more forthcoming. But there is no way they could have satisfied their former victims.
That brings me to Turkey and the Armenians. Nobody knows what the legal consequences will be when Turkey finally admits that it was built on mass murder. And so one sees that even people who advocate a more open attitude become more reticent when they are in a position of power where they could make a real difference.
In my opinion it is better not to pay too much attention to this genocide question. Normal relations between Turkey and Armenia are much more important. Turkey keeps holding its border with Armenia closed - even though it has promised the EU to open it. Instead of scoring cheap points with the genocide question the EU politicians should put serious effort in pressuring Turkey to finally normalize its relations with Armenia.
Once the two sides agree to treat each other like normal human beings an important barrier has been taken. It may still take decades before they formally agree on what happened during World War I but the discussion will be less tense and more open.
Don't get me wrong: it is important that Turkey one day recognizes what happened. However, I believe that there is very little that pressure can achieve in this respect: a favorable international situation (no tensions with Armenia) and a leader who feels himself strong enough to make such a move are much more important.