Trialists come and go all the time. There always seems to be room for them. Do you have any source to support your contention?
There's a major difference between hosting a couple of trialists in June and bringing in trialists during preseason when there are already a billion players from USL/draft/academy in camp. You're asking me to source... what? Basic math? There are a limited number of coaches and a limited number of people participating in a drill/scrimmage/game at one time. The more players you have, the more you have to spread around opportunity and the more players coaches have to pay attention to. There's a reason teams in all sports cut players as preseason goes on. It's because the coaching staff wants to narrow their focus to a limited group of players, and it's a lot easier for 5 coaches to analyze 24 than 40 players.