The point of Unicode is to try and be the One True Encoding and remove the need for all other language encoding standards, so there's never any more "mojibake" or cross-border compatibility issues. In order to do this, every feature that has existed in other encoding standards has to be supported in Unicode, or else people will stick to that other encoding in some circumstances.

With emoji specifically, they were popular in Japan dating all the way back to the 90s, via carrier-specific encoding standards. The lack of emoji support in messaging was a reason that the iPhone and Android were slower than expected to take off in Japan, and so Apple and Google asked the Unicode Consortium to add emoji support, so they could have this feature on their phones while sticking to a universal encoding standard. IIRC, the Unicode Consortium was actually hesitant to do this and didn't want to be involved with standardizing pictograms into Unicode, but eventually relented.