The FAQ mentions multiplayer, so it's clear the devs know a lot of players want it. However, multiplayer is one of those things easier said than done, and the devs would need to:
- make sure the connection is timely and fast, as the game is in real time;
- solve problems when the above doesn't happen;
- make sure the simulation works exactly the same in both instances, in a predictable enough way;
- deal with the increased CPU and GPU requirements from having 2+ players interacting with the environment at the same time;
- solve mechanic issues like what to do when two players interact with the same item in incompatible ways;
- solve balance issues that come from two people sharing progress together;
- find solutions for a bunch of stuff I didn't mention because I'm no dev.
So as you can see it's a huge undertaking, and can take literal years of development time, specially for a small studio like Monomi Park. Odds are the devs are busier making sure the foundations of the game are rock solid, before even thinking on multiplayer.
To give you two examples on how hard it is, it took Klei Entertainment two years from their main game (Don't Starve, 2013) to the multiplayer version (Don't Starve Together, 2015); Mojang had a bad time with multiplayer too, as up to Minecraft 1.2.5 it required them to develop two versions in parallel, and past 1.3 multiplayer introduced a lot of bugs into the singleplayer version.
TL;DR: as much as I'd also like to see Slime Rancher multiplayer, I think it's better for us to not count with it. If it comes it'll be a pleasing surprise.