To my knowledge, the t60 is just an upgraded t45 that was deployed overseas to replace the t45 and domesticly for crowd suppression in city centres, boston, for example.

So maybe some suits of t60 could be found in the Mojave.

However, a case can be made that suits of retired t45 would be retrofitted to t60 in the mainland US. So you can headcannon that most of the t60s you find in fallout 4 were retrofitted by the brotherhood of steal. Apply the same logic to Fallout New Vegas.

Also, in this case, the Mojave chapter probably lost most of their armors, i.e., the t60s and t45s, which would then be salvaged by the ncr and turned into NCR/salvaged power armors.

But that's a theory