Most FfH fans agree (According to a Poll long ago) that the darker terrain is better for a dark fantasy setting (and in general), the method of implementing the second set of terrain in the code was somewhat clunky, and it inflated the download size for something which was only utilized by a small minority, and was actually quite simple to overwrite the base files with (since that small minority ALWAYS wanted to use BM), so could be downloaded by other means.
Further, FF has added new terrains which Blue Marble doesn't have matches for. Placing Blue Marble in for the terrains which it DOES have makes the few it DOESN'T have stick out horribly.
So end picture is that it just wasn't worth keeping in the code. Honestly it's surprising Kael placed it in as a gameoption in the first place, but he might have already been looking at Scenarios at that point and contemplating having some of them look different by utilizing Blue Marble.