How does Minecraft do cross platform?
The short answer is that they're a company of about 600 and have the backing of Microsoft as a parent company. There are far fewer developers that work on Stardew (1-3 depending on the update, plus additional work from other companies on the console and mobile ports).
One thing that crossplay requires is that updates for the game need to be synced between platforms much more closely. When Minecraft puts out a new update, it has to be ready on every platform that supports crossplay and released at the same time, otherwise it would break crossplay. My assumption is that because Minecraft has more developers they can work on updates on multiple platforms in parallel, making this system possible without increasing developement time between updates.
Stardew's current release schedule where Windows, Mac, and Linux get updates first followed by the consoles would mean crossplay would be broken for a period of time. The PC/Mac version of the game could be held back until all the console version were ready to avoid that issue, but it would delay updates for anyone on those platforms. And switching to a system where all the versions are developed in parallel would be, I assume, more expensive.
