Tales From The Borderlands was "perceived as a failure" internally by Telltale
This is probably why there hasn't been a sequel.
Tales from the Borderlands is widely seen as one of Telltale's best works by fans and critics alike. And while many decry the company for opting to go with a second season of Minecraft: Story Mode over other IPs, it turns out that fans simply didn't pull their weight on the sales front despite the game's near universal praise. Several of the game's developers gave an extensive oral history to Campo Santo Quarterly, and the whole thing is quite interesting.
From trying to live up to the success of The Walking Dead and legitimizing the idea of a shooter IP meshing with a narratively-focused company, there's a lot that went on behind the scenes. But what's most important is that the game simply didn't sell well, and as Game Director, Nick Herman put it, the game Tales From the Borderlands "was perceived as a failure" by Telltale.
This would seem to fall in line with reports that Telltale doesn't have any plans for a sequel to Tales From The Borderlands. While it's easy to criticize a faceless company for making decisions that fans perceive to be dumb or irrational, Telltale is only following the numbers, which in turn means they are following the fans.
If you want more Tales From The Borderlands, get out there and buy it.