CMYK has a maximum 16,000 colour combinations. RGB has 16.7 million combinations.
If you do the math 16.7 million – 16,000 = 16,684,000 RGB colour combinations that aren’t possible to create using the 4-colour CMYK print process.
It’s standard practice to design logos in CMYK for print and then convert them to RGB for digital use to guarantee that you’ll have maximum consistency when reproducing your logo in print.