Genocide is different than "democide" (a term coined by Rummel). Genocide requires that people are exterminated for an
indelible trait. Gypsies would be genocide. Jews are genocide because they were treated as a
race, not as people who voluntarily held a set of beliefs. This distinction dates to the Spanish Inquisition. The Catholics in the camps were not there for being Catholic (Hitler, after all, was a Catholic as the church didn't see fit to boot him out, because he didn't do anything bad like some Nazis who WERE kicked out of the church (the crime being a marriage to a non-Catholic, which is clearly worse than mass murder
)).
Political enemies being exterminated is also not genocide.
The Soviets murdered more than the Nazis did (as did the PRC), but again, not strictly genocide (the communist hallmark was always the really random nature of their mass murder).
A focus on the jewish aspect is unremarkable in the US. There are far more jews here in the US than Gypsies, for example. Catholics were not persecuted by Nazi germany, in fact many of the persecutors were themselves Catholic... Nor is there a large block of former Nazi political prisoners around. So when seeking the museum, and you start hitting people up for money, who will you ask? From a pure marketing strategy to focus the fundraising on the primary group exterminated for an indelible trait makes plenty of sense.
Anyway, to be fair, the point was to focus on genocide, not merely mass murder (ie: others who were exterminated in the camps). If the focus was merely "death by government" then I'd expect the vast majority of the museum to be directed towards the communists, and their vast depredations.