From my (limited) experience with UK undergrad courses, it's more that stat mech and thermo were taught, but stat mech has been dropped/made optional. It's somewhat understandable, because a lot of interesting things have happening in other areas, e.g. quantum mechanics, astrophysics, etc. and the length of a degree hasn't changed. It's just one of those ironies that stat mech may have been ahead of it's time, and with cheap computing power it becomes interesting again (see molecular dynamics).

Still, when i took stat mech largely by accident (i enjoyed that lecturer), it felt like a lot of things came together which i had previously just accepted in thermo, and other fields, too (e.g. early quantum mechanics, from what basis did they start). Hence my opinion to why it's underrated (you also hear very little of it in text books - it's just not "cool").