Minecraft Java and Pocket Edition differences [closed]

Why are Minecraft: Java Edition and Minecraft: Pocket Edition so different in game rules, mob behavior, redstone mechanics etc? It's really frustrating when I search, for example, for a good iron farm design, find one and make it on PE just to find out that it doesn't work. Then I find a huge design for PE which I don't want to build because I'm lazy.

PE was never a branch of JE - it was a complete rewrite from scratch in different language, for different platforms, and initially vastly simplified in relation to JE - it was meant to run on definitely underpowered at the time smartphones. It was really bare-bones, probably simpler than Minecraft from times of Alpha.

JE from the beginning had the benefit of a fairly strong PC so it could afford quite a few liberties PE just couldn't.

Ever since, the development of PE tried to bring it somewhat closer to JE but it's still running on underpowered devices, every feature still needs to be reimplemented from scratch (different programming language!) and many idiosyncrasies of each conflict with each other and are kept for 'legacy' reasons, so for example neither will PE adapt quasi-connectivity nor will JE drop it.