Why are testosterone levels elevated in the morning, particularly among males?

Diurnal rhythm: Why? Because that's how we are made. Sorry, I don't have a good answer as to why, but rather an explanation of what is, which is to say, testosterone levels follow a diurnal rhythm as you've noted, being highest in the AM and lowest in the early PM. That's why athletic commissions require testosterone levels be drawn prior to 10AM and throw out lower levels drawn in afternoon/evening.