Why doesn’t the ACLU have the courage to sue the government over the religious reference on US money?

The ACLU will sue local councils over minor infringements of the first amendment all the time. The other day, I saw a story about the ACLU threatening to sue some local council over a Christian flag in some war memorial. This is a very minor, almost insignificant violation of the separation of church and state yet the ACLU were within their constitutional rights to sue if the flag wasn’t removed. The religious reference on US money is a major violation of the separation of church and state yet the ACLU doesn’t do anything about that. Why not? Why pick on small communities, local councils and war memorials but ignore the most blatant constitutional violations? Is it because this will generate major public animosity towards the ACLU if they did something like this, making the ACLU cowardly bullies who only have the guts to pick on small communities?