What mamba said.
“In an update provided to the Grim Dawn Kickstarter project on April 18, 2012, Crate revealed that the core of the company consists of only two full-time employees, with additional work being provided by former Iron Lore employees when needed.[8]”
Also:
"Several months later on August 19, 2008 Crate announced the acquisition of the Black Legion intellectual property that has been in development at Iron Lore Entertainment before that studio closed. Iron Lore had attracted significant interest from publishers while pitching Black Legion in late 2007 but had not been able to survive long enough to close a publishing deal. Crate had hoped to pick up on the momentum that Black Legion had gained but due to the U.S. economic recession publishers opted to pass on a large project from the studio.[4]
After nearly a year of silence, Crate announced on July 27, 2009 that the company had acquired licensing to use Iron Lore’s Titan Quest game engine for their new action role-playing game project.[4] On January 21, 2010 Crate announced that the name of their new project would be Grim Dawn.[5][6][7]"
I suspect that Black Legion may have been being developed using the TQ engine and if they wanted to move forward with it that’s what they needed to buy.
With limited funds and staff it’s just easier to work with what you know rather than creating an entirely new engine from scratch or having to buy a much more expensive newer one.