I believe if everyone who was against the current oligarchy started placing guillotine stickers everywhere, like you might for a favorite band, people would feel less isolated in their distaste for the current system.
And in an ideal world those who the guillotines are for might realize just how much they are in the minority.
So why not use a symbol from a successful example? Why use a symbol from an example we should avoid repeating at almost any cost? I think it’s because most people see the guillotine as a symbol for retribution against the ruling class.