Why weren't the Hubble light-echo images of V838 Monocerotis supplemented by ground telescopes?
Adaptive optics does not work (or at least did not 10+ years ago) on ground based astronomical telescopes at visible wavelengths. No images remotely comparable to HST could be obtained for faint$^*$ objects from the ground in the BVI wavebands.

I checked for papers discussing adaptive optics at other wavelengths and couldn't find any. The field of view is a factor. Back in 2003 there wouldn't have been instruments that could have covered V838 Mon at IR wavelengths with NIR AO observations. It can just about be done now at Gemini South, but field of view is still an issue for AO visible wavelength observations.

$^*$ NB "Lucky imaging" or speckle imaging can work from the ground for resolving binaries etc, but is not so good for imaging faint, extended objects. Having said that, I have seen some images of things like planetary nebulae taken with Lucky Imaging, but that is now and that was then. Nobody was doing it in 2003.