To Jack: I didn't mention English and Spanish cowboys because they weren't (as) germane to my point. And neither did I say that the largest percentages of immigrant cowboys were Irish and Scottish, simply that large percentages were. (Though from what I've read, I believe the numbers of Irish and Scottish immigrants among the cowboy population were larger than the number of English ones.) So yes, one might have expected modal contributions from both English and Scottish cultures as well—though not much in the Lydian line.

One reason for these percentages is that the Scots and Irish arriving in the East faced severe exclusionary biases that the English did not—recall the blatantly hostile "No Irish Need Apply" signs—forcing many to seek jobs in other parts, and droving talents from the old sod adapted well to cowboying. Scots and Irish also faced more severe economic oppression in their native lands (primarily at the hands of the English), so there was greater emigration from those parts during the period which encompassed the golden age of cowboying.

Proportionally, more cowboys came from the South (especially as ex-Confederate soldiers) than from the North, and the British/American modal tradition remained strong in that region—from mountain ballads to banjo tunes to shape-note singing. This also would have affected the modal balance in cowboy songs.

As for the Spanish influence, it's inseparable from cowboy life, which mostly evolved in Texas (and to a lesser extent, California). But for much of the trail-herding days, the cowboy enclaves remained fairly segregated into whites, Mexicans and blacks. As the songs were mostly exchanged around the campfires of segregated camps, well, draw your own conclusions on cross-pollination.

Who knows, maybe my mythical cowboy singer was a Rom: Slovak Joe the Wrangler...