I think he did not support the RTWL until now because he had other legislation to get passed. If he tied a Right to Work Law with his attempts to reign in public sector unions, he might have failed at both.
President Reagan, had a simple strategy, pick one or two big issues at a time. Governor Walker has done the same.
He also allowed the legislature to take the lead on RTWL on the advice of legislative leadership, which is something the anti-Walker forces on FR choose to ignore. Politics is as much about tactics and strategy as it is about making speeches and tossing raw meat to the base, and Scott Walker has proven to be an exceptionally good political tactician.
A lot of political capital had been spent on the previous fight...doing too much, too fast can damage the overall momentum. After winning again and with a larger majority in the legislature, there was more political capital to spend.