Cause if you do not stand up they will take everything from you and to use a quote slightly out of the original meaning, but really just speaks volumes to this and it was in reference to the smart people of germany for not standing up to the nazis:
'First they came for the communists, and i didnt speak out because i wasnt a communist. then they came for the trade unionists, and i didnt speak out because i wasnt a trade unionist. then they came for the jews and i didnt speak out because i wasnt a jew. then they came for me and there was no one left to speak out for me.'
…..if you do not stop them at this first step, then they will take everything, from everyone. just because no one stood up.