Why are some RGB colours difficult to match?

RGB is additive and made up of light, where the more colours you combine the brighter and closer it gets to white. The fewer colours you combine, the closer it gets to black. This explains why it's possible to get bright, neon colours in RGB. Your TV, computer, and phone screens work this way.

CMYK, on the other hand, is subtractive and made up of ink. The more colours you combine, the darker and closer it gets to black, and incoming light cannot reflect as much off of the substrate.

The example below uses RGB colours and shows the difference between the printed artwork on the bottom versus the digital artwork on the top.

RGB Example

Related article: Can you print RGB colours?