it is in Genitive because "ждать" means waiting and therefore it is waiting FOR something. The preposition FOR isn't there because the verb Ждать doesn't need to use prepositions (as far as I read it before). However, still the wait is FOR something so what comes next in this kind of context has to be in Genitive. Remember: "масло ДЛЯ салатА"......?
In your sentence, "waiting" continues to the present, or very recently.
In "We were waiting", the action was in progress at a particular time in the past, for example "We were waiting for the night, bu the mayor told us to start the fireworks". The particular time is when "the mayor told us". A simpler example: "We were waiting for the night at 8 p.m.
For other users who just began this exercise on the past tense: The past tense doesn't distinguish by person like the present tense (me, you, he, etc). Instead, it distinguishes by gender, so there's a masculine form of the verb for a m. subject, etc... Also, it distinguishes by number, but only between singular and plural!