Why is there no extensive Standards Body overseeing ISAs, Bitcodes, Code Representation, etc... as there is in the case of Unicode
Unicode is a suitable encoding for most scripts because it includes characters from all the scripts. The analog would be an instruction set architecture that includes instructions from all processors. This would be huge and impractical to implement whether in hardware or in software. It would even be contradictory: how do you reconcile an architecture with 16-bit words with an architecture with 64-bit words? an architecture that allows unaligned word accesses with one that doesn't? an architecture that guarantees the presence of an MMU with one that guarantees that addresses used by software map directly to addresses in physical memory? an architecture that guarantees that all memory accesses take exactly the same amount of time with one that has memory caches?

A realistic ISA can't combine what all the ISA do. You have to make choices, and these choices depend on your objectives. If you're building a processor for real-time processing, a guarantee that memory accesses take the same amount of time is crucial. If you're building a processor for fast numerical or symbolic computation, caching memory accesses is essential for performance. A good instruction set for general-purpose computing is not the same as for graphical rendering, and machine learning calls for yet another approach. There is no one-size-fits-all.

Why isn't there a standard model of vehicle? Why are there so many different models of cars and airplanes and bicycles and motorcycles and helicopters and submarines and so on?