- When they proposed solutions they were called out as being "politicized". Both left and right-wing saw them as biased (in the other direction of course).
- When they didn't propose anything, they were called out as being gutless (as you said), lazy, whiny...
- When leaders tried to step out they were criticized as opportunists trying to kidnap the movement for their personal interest.
- When assemblies were formed, they were criticized as slow and not effective (which they actually were).
Of course all these criticism were promoted from the media to form mass opinion. In case you didn't like a statement, you had the contrary to criticize 15M as loud as you wanted.
In the end nothing happened.
"THIS PROTEST MOVEMENT SUCKS." Nothing more, nothing less. A variety of particular reasons are given for why, many of which are mutually contradictory. From this we can infer that the point was never to have a civilized discussion about the protest movement, its goals, and its methods, but instead to just flame it to death.
This is the mainstream media equivalent of sitting there going "lolol dumb faggot" until the tripfriend shuts up and goes away.
We're back where we started: 15M didn't succeed because it tried to fight State with State. It's counterintuitive, but you can't vote politicians out voting some other politician in. Different dog, same collar. And if you become a politician yourself (and follow the system's rules in the process), you're fighting the State with State (which IMHO is pointless).
Also: you focus on politician's blame which I think is not the problem. I think it's not the person to be blamed, but the State. The institution itself.