I know there are varying reports especially on Reddit, but here in California I see a ton of raid activity as early as 7:30AM, and almost zero activity after 5:00PM.
(1) It may be summer, but people still have to work. So having raids happening (and I'm mostly talking about 4+) anytime between the ol' grind of 9-5 isn't going to see much activity. Unless of course people start ditching work, and being irresponsible (which I would never do) >.>
(2) Its summer... its hot. So leading into the evening is when MOST people are heading out. The larger hubs, malls, strip malls, etc. Don't even see much activity until the dinner crowd comes out.
(3) Raids (legendary) in the middle of the day. Obviously I am not unique in wasting raid passes, only to find that no one or too few are there to participate. But assuming they weren't working on tools for us to see other trainers, or communicate to collaborate/cooperate, an easy fix would be opening up legendaries (which are gone now?) during times when most people are out and about.