There are numerous posts on this on the discussion thread which go into detail. You can also read about it in the lesson info. It's basically because oidhche is a feminine noun. All feminine nouns cause lenition.
It's an interesting word because it is unusual in that no-one knows where it comes from. It is only found in Gaelic, Irish and Manx. Most European languages (including Welsh) use a word beginning with n, so where this apparently unrelated word comes from is baffling.