#1 reason = Umpires! LOL J/K
While I understand the purpose of the question, I don't agree with it. My team lost Thursday. To be blunt, we just didn't have the athletic ability of the team we played. Out of 9 positions, we only had one athlete better at their position than that team.
The reality for me is most often when I think of losses, I think that the fault is mine. I do my best to make sure that I never leave anything to chance. Still, I fall short. So, I have to think about what I can do better in practice, with the lineup, the mental game, ... So, for me there isn't "one thing" that sticks out. To be a good coach, you have to make sure that you are always learning and so, you make sure that you prepare your team the best to win. JMHO!
Today at a clinic they asked what is the #1 reason why teams lose, and I answered "Because they score less runs that the other team." which was the wrong answer. The right answer was "because of how they field ground balls & throw", I didn't know that before today.
If we are talking teams of similar talent & skill level, I totally agree. You make all the straightforward plays, you are putting yourself in a great position.
It is also why I promote in early rec and younger age groups that the absolute best fielder plays 2B. There is a lot of balls that go to 2B and they MUST be outs. In the early ages, outs from SS and 3rd are on most teams are 'bonus outs' and are just as likely to be extra bases on over throws as they are to be outs but no one is surprised when it happens. When you make that easy 2B-1B error that regularly turns into the 'big inning'
You'll usually lose when you play above your level. A few years ago I had a team that was in all honesty a B level. However we had a schedule of nothing but A, Gold and a couple opens. (My theory was "we won't get better playing low level competition" ) We lost the first 15 games straight. We would compete and stay close but we'd always come up short a run or two. Our first win came in an open tournament against a sanctioned B level team...
Team became disfunctional and parents non stop chatter ate away the team. That season dissolved right before our world series trip. I bought into a B level all sanction tournament in TN... We would have probably killed it there, but the damage was done and 4 or 5 players suddenly couldn't make the trip!
Point is to find a balanced schedule that is challenging yet not so tough they quit on you. The losses mounded up so high that they saw themselves as poor players or "we suck" so what's the point attitudes...
Funny thing is now almost all of them are their current teams stud player several are freshman in HS now and start on the varsity team.