M'Harry discusses all-corn and all-rye (except for barley malt) mash bills, but he was based in Pennsylvania of course.
He preferred a mixed mash too, though, one that is similar again to modern bourbon recipes.
I think the answer, Mike, is simply that people found that the mixed grain recipe tasted best. As Chuck said, all-corn would produce a milder product (although for new whiskey that might be a relative thing), and all-rye would produce in many cases a rougher, too-strong whiskey. The modern bourbon mash formula was probably worked out very early (late 1700's).
In Pennsylvania though, aged rye whiskey had a market and it may have been simply the fact of rye (as a crop) being traditional there. The mashbills however were not always 100% rye (exclusive of barley malt). Some were all-rye, e.g., Baltimore Pure Rye was 98% rye, the "rye-e" rye, it was dubbed. Probably they used malted rye and a little corn or wheat (Irish whiskey, pure pot still, used to be made with malted and unmalted barley and 1-3% rye, maize or wheat). But some distillers would have used I am sure 2/3rds rye 1/3rd corn (plus barley malt), in fact I seem to recall seeing a bottle of Finch whiskey from Pennsylvania, from around Civil War time, that advertised that mash bill on the label (or was it the reverse!?) and Michter's Original Sour Mash was another variation (essentially half corn and the rest mostly rye with some barley malt).
F.X. Byrn, writing in the 1860's in Philadelphia, gives varying recipes for rye whiskey which involve 70%-80% unmalted rye and the rest barley malt. Perhaps by this time, the industry was distinguishing itself in that State by making all-rye whiskey, to set itself apart that is from the Kentucky bourbon industry which was growing by leaps and bounds. But that some rye + corn mash whisky was made in PA at the time seems undeniable and indeed bourbon was made there until the 1970's.
I think really what you found in Virginia and Kentucky became that region's preferred taste with some rye whiskey being made to cater to the older taste for rye-predominant beverage, and the obverse developed in Pennsylvania and Maryland (with Maryland being more blended-oriented), where rye was dominant in the mash. This would have made sense from a business specialisation point of view, the industries would have segmented in this fashion so that Monongahela could retain a market, Bourbon could retain and grow its market, Maryland could retain its (always smaller, more tenuous) market with its rye and blend variations.
Canadian whisky, the least flavoursome of the rye-derived whiskies, ultimately removed most of the market for American straight rye which is unfortunate, it was a kind of usurpation although very much with the concurrence of the consumer (who could have resisted but didn't bother).