Note: this is only my guess, but it seems quite plausible that inhabited areas have been well-sampled for a long time, but some combination of

1) automated stations and satellite communication making it much easier/cheaper to sample remote locations

2) use of whole-globe numerical weather models which require complete initial conditions, even for regions where nobody much cares about the output

3) increased interest in climate research

would lead to a greatly increased number of Alaskan stations operated by NOAA (and of course corresponding moves by other national agencies)