Why would they include a DRL they finally admitted they had to start studying in 2010, in a 2008 LRT plan?
Transit City wasn't just an LRT plan, it became the City's Transit Master Plan. The fact that it was all LRT lines doesn't change that.
The answer of course, is that the cost of the DRL would more than exceed all 6 of the surface LRT plans in the Transit City plan. And that with Transit City mostly finished or under construction by 2016, they could then turn their head to a very expensive line, which wasn't going to be affordable quickly.
TMPs frequently include projects that are medium or long-term projects. I can understand that the LRT lines were intended to be "quick wins", but there's no reason, aside from not caring about it, to exclude it from that plan. To say "well at least they started studying it in 2010" completely ignores the fact that the architects of Transit City completely ignored the DRL back in 2008.