Because Islam mandates, in Quran, to all the follower to promote the good and work to eradicate the bad. Islam is a way of life it is not confined to standing on musala and saying 5 times of namaz and you are done. Islam gives you a complete framework and touches every aspect of your life.
Dress code has been mandated in Quran , which is the ultimate code of conduct for a Muslim.
If I see a Muslim breaking that code of conduct it is my duty to let them know , assuming they do not know , once I have told them and they do not care , I would go my own marry way.
That being said Islam does not mean to make your life difficult and wants you to enjoy the bounties of Allah. You are allowed to wear or not wear whatever you like or do not like in the privacy of your home. Public display of your body parts which are supposed to be covered is not allowed in Islam and all Muslims are bound to honor that code.