What concrete factors led to The Irishman performing poorly at the box office?

I think we can go straight to the Wiki page for our answer:

The Irishman received a limited theatrical release on November 1, 2019 in the United States.[75] As part of the continuing tensions between the film markets for direct to digital streaming and theatrical releases and distribution of films, several theater chains protested the policy of Netflix for the release of Scorsese's The Irishman. The film did not play at the theaters owned by AMC, Cinemark, Regal or Cineplex, because the "four week progression to SVOD remains unacceptable to those chains."[75] It was previously reported in February 2019 that Netflix would possibly give the film a wide theatrical release, at the request of Scorsese.[97] The heads of several theater chains, including AMC's Adam Aron, who refused to play Roma the previous November, said they would only be open to playing The Irishman if Netflix "respects the decades old theatrical window, that suggests that movies come to theaters first for a couple of months, and then go to the home."[98] Two major chains offered to exhibit the film if given an exclusivity window of 60 days, approximately two weeks shorter than the typical window, but could not reach an agreement with Netflix.[99] The film peaked at playing in 500 theaters, essentially the most possible without the involvement of a major chain.[100]

So, since Netflix was so heavily invested, they wanted to remove the theaters from the chain. The theater chains knew this and mostly refused to play it because they knew attendance would be down due to the fact it would be streaming in a month. Therefore, with such limited release and for such a short period of time, the box office take was very weak.

Also, read the Financing and Budget section of the Wiki page for info on how Netflix became the main financier of the project, which is what set off the theater chains.