Here is a really good reason: besides the fact that shellac holds up well for this purpose anyhow, it can be dissolved with alcohol. (use denatured) Which means that it you get it anywhere else on accident, such as on the brake levers or if it drips down onto you wheels, or invariably seeps onto the handlebar, you can just wiped a bit of alcohol onto the spot and then wipe it with a clean cloth and your unwanted shellac is no longer present.