Because it was a sale. It was on for a few weeks. Guess you'll have to wait for the next one. This happens all the time, with all phones, with any carrier. When you see a "good deal", you better jump on it, because it can change at any time.
To be honest though, I don't recommend going with Tab L. The website doesn't show it, but you can get it on Tab S/M too. You'll end up paying a lot more upfront but you'll more than likely save money over the 24 months.