Because they don't commit to doing what it takes to service their customers come hell or high water. You need to have the postal carrier mentality....Neither snow nor rain nor heat nor gloom of night stays these LCOs from the swift completion of their appointed rounds. That and they tend to try to compete by having low prices. And most importantly, they don't have a contingency plan and/or cash reserves for when something breaks or when their income stream dries up when the drought dries everything else up. You might be able to run a household paycheck to paycheck if you have a steady job, but you can't run a business that way, because income and expenses are not fixed...they vary.
What would you do if you blew the engine in your Bobcat tomorrow? What if someone T-boned your truck. Would your lawns still get mowed? What if you messed up on your mix and killed 3 lawns and had to wholly replace them all? Or if you fell off a step ladder doing a shrub job and separated your shoulder? It's that type of thing that can put a small operator out of business in a heartbeat.