Because a popular revolution shouldn't be labelled a "US-backed coup", and labelling it as such strips agency from the protesters who made it happen. Getting the label right is important given that this is a critical part of the Kremlin's justification for the invasion: casting the current Ukrainian government as illegitimate, and stripping Ukrainians of agency and their identity.

The evidence that it was a US-backed coup is quite weak; from Nuland's leaked call, to Nuland handing out sandwiches to protesters, to the speculative opinions of Estonia's FM, to the few billions of investments the US has made into civic institutions since 1991 -- it's a little bit of smoke if you squint really hard and apply a good dose of confirmation bias, while simultaneously sweeping under the rug Russia's more aggressive colonial meddling in Ukraine (such as Yushchenko's poisoning).

Maybe better evidence will emerge later about 2014 as records are declassified (such as it did with Diem's overthrow), it's possible and wouldn't be surprising, but barring better evidence, it needs to be labelled the 2014 Maidan Revolution, or something similar.