Pretty much what Polish Eagle says. The technological gap would be too great for the most technologically advanced human powers to reverse engineer any Race technology, or to effectively use it if captured. Also, as opposed to 1942, There are many fewer technologicaly advanced powers to deal with, and they are all much more localized.
Earth would still be a somewhat special case for the Race, because even at 1848 levels, it would seem we are much more advanced technologically than the other subject races were when they fell to the Empire, and our planet is much less hospitable to the Lizards climatically, and as noted, the ginger issue remains.
Assuming low Big Ugly level resistance continues (which it probably would), the Race might still decide Earth was a poor investment. As described by HT, humans are much less likely to accomodate to Rule by the race and it is not likely humans would become regular loyal subjects like the Rabotevs and Hallessi. It would become gradually obvious to the Race that, if not kept under an iron thumb or exterminated, humans would eventually pose a risk to the stability of the Empire. Once they came to this realization in 200-300 earth years, the Race might decide to just cut their losses and leave, and nuke the place to make sure the humans never become space faring.