Why is print pixelated?

It doesn’t exactly look pixelated, in that it doesn’t look like it lost spatial resolution. It does look posterized, as if the colors and tones are not being represented correctly.

However, printing does involve a lot of variables, and if just one is off, that could ruin the print. Two computers should create the same quality print with the same software if the new computer:

• Has the correct, up-to-date driver software installed for the printer, in your case the Epson driver for the SureColor P800.
• Has the same Epson printer driver settings that were used on the other computer, including but not limited to the same color management settings, and the same Media Type setting.
• Has the same Photoshop Print dialog box settings that were used on the other computer, including but not limited to the same color management settings. If Photoshop Manages Colors is selected, then the appropriate color profile for the paper and ink must be selected.

Any deviation or forgotten setting can result in a very different looking print. When setting up a new computer where everything is freshly installed, it’s easy to forget a buried option that was set a certain way on the old computer and hasn’t been changed in a long time. On the new computer, it’s worth double-checking all the print settings in all the subpanels and submenus to see if a setting is set to some default that you need to change.

What are the actual specs of the file? Dimensions and Resolution.