Why weren't new episodes of Elementry,Person of Interest and Scandal aired this week?

The regular television season for a show picked up for a full season (22-24 episodes) usually goes from mid-late September until mid-late May so they can't air new episodes every week. There are repeats interspersed in between new episodes and some shows even decide to take month or more hiatuses where they show nothing but repeats so that when the show comes back from a hiatus they can show a lot of new episodes in a row.

The shows you listed were showing repeats this past week. Perhaps they will be new next week.

There are only 22-24 episodes in a TV season,so they have to take breaks in order to stretch out the season to make it last until May.