Because not a single title in the advanced wars series was selling worth investing in. Capping out around 700K for the first entry. And critical quality gradually went down between entries. A lot of japanese devs have a bad habit of sequelizing despite lost cash in a vain hope of making a series mainstream eventually. See armored core, F-zero, Darkstalkers, most megaman sub series, and a lot of others for reference. It only works once in a blue moon, ironically it happened with Fire Emblem several years after the last advanced wars. Realistically speaking, and it pains me to say this, more often then not in these scenarios, it's best for the dev to close up shop on said series and try again with something else.
Nintendo stopped caring about F-zero because people stopped caring about F-zero. We all like to pretend it was this huge thing, but by the end of it's run F-Zero climax reportedly sold less then 10K copies in it's opening week in japan. That is laughably horrible for what is arguable one of the series strongest entries. So no, they didn't stop caring, people did.
I've had this argument for years. Camelot didn't stop producing Golden Sun games because the sports games were more profitable. Because if you go into the sales data FOR those sports games, only 2, Mario Tennis Aces, and MArio Tennis 64, have had sales comparable to that for the first golden sun. The problem isn't that the series isn't profitable. The problem is that all the talent that made Camelot an excellent RPG developer like they were, left. A long time ago. See Dark Dawn for evidence. A lot of the orignal team had left and a sports game dev now had to make an RPG. We all know how that turned out.
At the end of the day, Nintendo is a business. Their primary objectives are to make money, pay employees to produce products which will make more money, and stay sustainable for as long as possible. Behind every fallen IP there is a story. More often then not it boils down to money. As sad as it is, that is the cold reality of the gaming industry. If you were a billionaire, and a group of people you didn't know asked you for millions to produce a product statistically proven multiple times to not be worth the initial investment, you'd have to be dumb.