What could we possibly be waiting for? Saving money is the only answer I can come up with.
Just playing devil's advocate, but up until yesterday, you could argue that they were waiting to see where Barton was going to sign. Barton was kind of the upper tier of the buy out market. And you don't know if other players are waiting. It may not be the Celtics that are waiting.
The only other argument I can offer is that they have made up their mind that they are going to sign Tony Snell. If that is the case, you could see the argument that they may as well keep him in G-League for as long as is needed to get him ready and for as long as he is going to be in Maine anyway, no need to sign him to an NBA contract and pay the tax on the NBA contract. I don't know that this is the case, just another possibility.
Otherwise, I agree, just sign someone already, now that Barton is off the table.