According to TSN Analyst Dave Poulin, who was a guest on TSN1050's First Up today, the Leafs did not buy out Murray during the initial window because they wanted to use him as leverage in negotiations with Ilya Samsonov. Poulin was asked about Murray being placed on LTIR and the possibility of him being bought out and also revealed why the Leafs have waited to do so.
"I'm not sure Murray would qualify for LTIR right now. He came back and was healthy and ready to play at the end of the season, so that's a complication on that. The idea of not buying out in the first window is the fact he's an NHL goaltender and that was leverage against Samsonov in that you had an NHL goalie on your roster. Once the agreement is made with Samsonov, then ideally the Leafs would buy out Matt Murray," said Poulin.
Thinking about it, Poulin's reasoning makes a lot of sense. The Leafs knew they would have Murray under contract next year so that if they were unable to agree on a new deal with Samsonov or if they felt the arbitration award would be too high, they would not have to panic and overpay for him. As Poulin said, once they sign Samsonov, we should expect them to buy out Murray and free up an additional $4M in cap space for the upcoming season.