The Switch and XBox One aren't really competitors to the same degree as the Xbox and PS4, and the games in this announcement aren't the sorts of games that sell a particular console. Rather, they're the sort of stuff you can sink time into between your purchases of AAA exclusives.
Annoucing this cross-play is a cheap win-win for both MS and Nintendo: For Nintendo, it solidifies that in the right context, the ultraportable Switch is a comparable platform to the cabinet-bound boxy consoles that are still current in their generation -- a marketing victory that handily exceeds the reputation of its predecessor, the Wii U. For Microsoft, it legitimizes their cross-play message by spreading beyond platforms that they both happened to own, and makes Sony look bad.
I think you're underestimating the popularity of Minecraft and Fortnite. There are absolutely people out there who only play Fortnite and nothing else. But you are right in that the Switch doesn't compete with the XBoX the same way the PS4 does (Nintendo consoles always played by their own rules). But Microsoft has already said that they'd be fine with crossplay with PS4, it's Sony holding up the line. Epic already accidentally (or so they say) enabled XBoX and PS4 crossplay once before for a short amount of time [1], so the work is already done, too.
Why would a company care about people who only play Fortnite? They’re not going to be buying anything other than those games, and when the fad dies away, they will too. It’s one thing to cater to casual players, who will buy lots of different games, another to Madden/COD players who will buy yearly installments. Pandering to s single fad that will be replaced within a year though is crazy. People will get tired of Battle Royale, just like they did with Hero shooters, and the fads before that.
1. Because it gets people on their system, who will then buy more games when the fad does eventually die out (which let's be clear won't be anytime soon). If someone playing Fortnite on PC asks their XBoX friends to download and play with them, now a bridge has been formed between the Fortnite only PC player and the XBoX player. It's only a few conversations away from "Hey, I got a 4K TV, let me get an XBoX so that I can keep playing Fortnite and all these other games with my friends."
2. Because it garners good will. Within the last few years, more and more companies are realizing that doing good by the consumer can still be profitable. Sony's the dominant platform now, but that can change within a year. If Microsoft is over here saying that their games are going to be backwards compatible forever and the consumer knows, through history, that Microsoft allows you to play with your friends no matter where they play, that's a substantial incentive to get the next XBoX over the next Playstation.
3. Because, and this is the simplest reason, why not? The work is already done, all Epic has to do is flip the switch. It requires no extra work on Sony's end, just willingness to allow their console to talk to other consoles. The way this is going, this is going to be net profit negative in the long run, with more people jumping to XBoX if they know Microsoft plays well with others.
Annoucing this cross-play is a cheap win-win for both MS and Nintendo: For Nintendo, it solidifies that in the right context, the ultraportable Switch is a comparable platform to the cabinet-bound boxy consoles that are still current in their generation -- a marketing victory that handily exceeds the reputation of its predecessor, the Wii U. For Microsoft, it legitimizes their cross-play message by spreading beyond platforms that they both happened to own, and makes Sony look bad.