The purpose of mixers is good, they help you to increase your privacy but the problem is that probably majority of users use them for illegal activities. Even if 1/3 of their users use it for illegal activities, it's enough to convince people that mixers are bad for everybody.
~snip~
I would even say it this way - even if 99% of people use mixers only to protect their privacy, and one client appears who wants to use mixers to launder coins he acquired illegally, that is enough for any government agency to label mixers as something bad that should not exist. What mixers could have done perhaps was to somehow protect themselves from such things by monitoring what is happening and blocking such transactions - but given the way they operate, such clients may be their main source of income.
Just as any tool (hammer, axe, chainsaw...) can be very useful, it can also be destructive - we must agree that privacy-enhancing tools have their pros and cons.