Why is the current atmosphere made of nitrogen (75.5%)?

From what we know, the Earth's original atmosphere was not made of nitrogen (or oxygen, or carbon dioxide), but of gases such as hydrogen and helium. These would have been lost to space early on, because the Earth's gravity was not strong enough to hold on to them. The present-day oxygen came from organic sources, as you said. Here's what Wikipedia has to say about this newer atmosphere (emphasis mine):

The next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases, was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids.

The page attributes that passage to this, which concludes that volcanoes are one potential source of Earth's atmospheric nitrogen (see "Volcanic Atmospheres").

Also, I found another page that supports the outgassing theory:

The original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials.

So it appears that the primary sources were volcanoes and extraterrestrial bodies.