Why did the Western countries ignored the Rwandan genocide, when they knew this was happening?

It is nobody's business to interfere in a civil war.

It wasn't politically expedient for Western countries to intervene in the genocide, which took place in the context of a 4-year civil war and was triggered by the assassination of the Rwandan president. The risk of military casualties, coupled with the lack of any obvious strategic or economic imperative, would have made such an intervention very difficult to justify in the eyes of the public. Even a few dozen soldiers returning home maimed or in coffins would have led to an outcry that young men's lives were being sacrificed for the sake of a country that at the time few people had heard of and fewer still would have been able to spell. Furthermore, the genocide was the culmination of a power struggle going back to the 1950s, and even if a joint operation by Western countries succeeded in quelling the violence in the short term, the underlying tensions wouldn't have gone away, and it would probably have required a permanent military presence to prevent a recurrence of armed conflict. 

Because violence, tribal wars have been going on in Africa for hundreds of years. The modern versions just mean a higher death toll

A few factors: it was far away, violence in Africa is not uncommon, it would have been very difficult to fix, and there is no oil in the area.

Every day, somewhere, there are similar happenings. There are no resources/organisation or support to deal with them all. It's only down to a few individual countries to take any action.

The entire international community looked the other way. Btw, people are presently being killed in Mozambique. What are you doing about it?

They did not ignore it. The West just did not do enough against that. However, many people might have complained: look, the West interferes again...possibly for their own benefit (post-colonialism)....typically!

I know this especially from Muslims who want the West to get out of the Middle East and yet, when a crisis looms want the West to solve it by removing unpopular local aggressors (dictators, insurgency groups like the IS etc).