Undoubtedly there is/was a reason, or likely several of them. While I have never seen anything of an official nature on the subject, I have had a couple of ideas wandering about - one of which is the possibility that prospective purchasers of surplus locomotives were not interested in the product of a defunct builder (as Grant effectively was from c.1886 on, and in fact from 1893); there is no immediately apparent physical reason for the Grants to be preferred over the Baldwins, such as larger heating surface or any such thing as that (the Grants were, after all, built to essentially the same set of plans as the Baldwins - the only reason that Grant got the order in the first place was that Baldwin was too busy to supply the D&RG with as many locomotives as it wanted, as fast as it wanted them), so the reason for the apparent prejudice would seem to lie elsewhere.
The lot of locomotives that went to the Utah company in 1886 was not a sale as such, but rather a transfer to the D&RG Western in part settlement of the litigation arising from the lease controversy, receivership, etc., the Western basically picking and choosing what they wanted to have on their line; the impression is that the Grants were not liked on the Western, as no record whatever of one ever running in Utah has as yet come to light.
Much the same with the lot that went to the Rio Grande Southern - Mears apparently specified Baldwin, and since the D&RG then had a substantial surplus of narrow gauge equipment as a result of the widening of the main line in 1890, weren't in a position to try and dictate anything to a well-heeled prospective buyer-in-quantity.
After the RGS lot, there is no further reduction in Class 60 (to become C-16 in 1923/24) locomotives until 1926, when some 31 or so of the C-16 locomotives were retired (sold or scrapped), 15 of the Grant and 16 of the Baldwin; this wholesale retirement was one result of the arrival in 1925 of the class K-36 480 series 2-8-2 locomotives, which together with the 470 series of 1923, now handled almost all the 'mainline' freight traffic.
So, the reason seems to be one of chance driven by the desires of purchasers more than of any premeditation on the part of the Rio Grande. Or so it seems to me, and there you have it.
