If i had to take a wild guess, i'd say the DLC didn't sell nearly as well as with the previous games.
Costumes: In the previous games, the effect of the costumes on the score was pretty small, especially later in the game it really didn't matter what costume you'd wear, you could just use whatever you liked. But in One For All the costume makes a huge difference, and the difference gets larger the later you are in the game. If you don't use a costume with the appropriate image and/or type, you are likely to fail an event, so you can't use whatever you want. But even within the same image and type, there are hug differences in the costume's skills. This introduces balancing issues with the costumes: Do you want to make them pay to win items by making them strong and trivializing the beginning of the game? Or do you make them weakish, but also obsolete later in the game because other costumes are way way stronger? They chose the second option, probably also to avoid pay to win backlash.
Songs: This is simply due to stupid trophy design. There is one trophy to get all non DLC songs to platinum status. But this takes a huge amount of time, so most people will long long be finished with the game before they get the trophy (i got all girls to S rank and 5 hearts and i'm not nearly done with this trophy). This trophy actually adds an incentive to NOT use DLC songs, and if you are not going to use them, why buy them? Removing the trophy or simply change it to "get 20 songs to platinum rank", no matter whether it is DLC or not, would have prevented the problem. I didn't buy a single DLC song, without this trophy i would have bought several of them. And to make matters worse: it was pointless since i didn't get the trophy anyway. And i'm sure i'm not the only one where this happened.
Another thing to consider: Do we know how well the later DLCs sold in the previous games? After half a year, most people have probably moved on anyway and wouldn't buy any DLC even if it were there. Personally i didn't bother with the DLC of the previous games after i had finished the games (imas 2 after 4 or 5 months, ofa after the ex episodes). Maybe that's the reason why they decided to stop earlier than in the previous games.