This isn't surprising. The NBA is talking about phasing in revenue from the new TV deal beginning in summer 2015 instead of 2016, which is what the CBA would call for. If this occurs, Leonard's max deal could be $2-3 million more per year than if it doesn't. I'm sure the Spurs don't want to commit to a contract that could wind up being $15 million more over the life of the deal if the NBA changes the rules after the deal has been agreed to.
Besides, there should be no rush to get a deal done. He's a restricted free agent next summer -- they can always give him a max deal then if he earns it this year, or match any offer he gets.
The big risk for them, however, is if the salary cap next summer doesn't include any of the new TV money. In this case, Leonard might be tempted to just take the qualifying offer and sign a new long-term deal the following summer, allowing him to make perhaps $6-7 million more per year for his new deal. He would also not be a restricted free agent. It'd be a major hit in his 2015 salary, about $12 million, but he could make that up by year 3 of his new deal.