The fact is mentioned twice in the article with no reason given.
There is a lot of speculation about this in the comments:
1. Silver had something on Cuomo, and would not go down by himself.
2. The investigation was getting a little too close to Cuomo’s activities.
3. Cuomo didn’t want to be seen taking down Silver, so he pretended to close the commission, but actually turned the information over to the Feds for action.