There are a few factors keeping it tough.
First is that those greens are tough. They're sort of the opposite of most greens the Tour plays on, where if you get it on the right level/section you have a pretty flat putt. No flat putts on the small, extremely sloped greens at CHCC.
Second, it has been lengthened. That new 8th hole Doak's team built is 276 yards! #15 is 247 yards. Converting the members par 5 18th to a par 4 is nothing new, but #5 is also playing as a par 4 this week ... 526 yards of dogleg with a fronting bunker. These holes help to counteract some of the obvious scoring holes, like #1 and #3 which are drivable par 4s.
Third, the altitude and dry air does help guys hit the ball farther, but this is not always a blessing. It gets really tricky to try to dial in a pitching wedge from 180, as you try to factor in not only all the normal stuff that comes with hitting an approach (lie, wind, where to land it) but also try to factor in the altitude, which doesn't affect every shot consistently. Higher lofted shots don't get up in the air as quick, lower shots don't get the same increase in distance, etc.
And then there's the setup. The rough is very penal. It's not what most of here are fans of, but that's just what the classic courses are going to do to defend themselves. For what its worth, I'll say that yesterday sort of put the lie to the common contention that heavy rough only ever means one recovery shot option. Just around the first green, guys were trying flops, chunk and runs, hybrids, chips, basically everything but putting.
The course was also quite firm yesterday, which is pretty typical for this time of year in Colorado. My understanding is they got quite a bit of rain last night. CHCC drains well and Colorado generally is a place that can dry out quickly, plus the limited field means guys don't tee off early in the morning, but if the course is softer it will be interesting to see if more guys go lower today. I think some will. There were better scores than 67 out there yesterday. But all the other factors discussed above should still keep anyone from going really deep.