How come that the right is so weak in Greece after the economic collapse and the migrant crisis?

If by "right" you mean the nationalists, there are a few reasons. Leftists promise a better living and people are ready to take that promise even if that means more "refugees". Also poverty. Nationalists are not part of the system and often live a difficult life, so they can't raise a family or print thousands of posters as the leftists do. People are ready to sell out so they can get a job, as everything is controlled by the political system. In general, people everywhere prefer the easy life.

And people still vote traitorous politicians like these that prefer invaders? If there was even a little bit of decency these guys would be executed for treason.

It's similar to the two party system in USA. There are Democrats, there are Republicans, often there's little difference, but people vote one or another instead of trying a third solution. It's really astounding, but it's the way it is.