Production of industrial machinery like farm equipment requires massive factory infrastructure with significant supply chains. Getting the capital together for that is hard enough, but then the new company would be competing against the established John Deere, with the added (perceived, if nothing else) handicap of lenient repair rules.
An existing competitor to John Deere? I suspect it is for the same reason that sedans are essentially indistinguishable, style-wise: operating in the same environment (aerodynamics, interior volume and shape, etc. on one hand, the legal and cultural background on the other), they have all struck on the same optimizations, which include repair restrictions.
Now, if one does decide to try the alternative, and makes significantly more profit, you'll see a fairly rapid state change as most of them switch.
Market position and power takes time to be established. It takes time to shift. Even under extreme negative pressure (that is, "loyalty" is due far more to lack of alternatives than genuine delight or satisfaction) collapse can be a long time coming.
And that's before anticompetitive pressures are brought to bear (e.g., exclusive contracts or threats of retaliation against any of the above).
Lots of moats - financial, reputational, generational, technological, branding, performance, distribution, etc.