"I love glass cockpits but the more information you have to read
on the screens in front of you, I'd say that there is a greater
at least temptation to decrease looking outside the cockpit."
You're absolutely right, but depending on what you're doing with the airplane looking inside may not only be tempting but essential. It's all about task management as you begin to use airplanes for something other than boring holes in the sky.
The pilot in this particular incident may have been dividing his time between navigating through terrain trying not to misidentify his next turn point since one valley often looks like the next, or simply trying to plan his flight path so as not to hit that terrain. Many things could have justifiably diverted his attention just enough so he didn't notice the glider, that according to the report would have been difficult to spot even if he were specifically looking for it. Ditto with the glider pilot.
There are plenty of other hazards to aircraft beyond mid-air collisions that also require attention, and usually there is a job to accomplish as well. The trick is managing it all which requires training and usually experience to do well.