edit: phoog beat me to the short answer, but offers less background.
If you look up the 1 Train at the MTA website, you'll see that it does not stop at Cortlandt St as that station is temporarily closed.
After the September 11 attacks, a number of subway stations were closed. Their treatment in subsequent maps is presented clearly on Brian Abbott's blog:
- In the first maps issued after September 11, the closed stations were removed from the map entirely
- By October, most service had been restored, and suspended service indicated with dotted lines
- In December 2001, the closed stations were represented as grayed out
Most service was restored by October 2002, but the Cortlandt St stop remains closed, and so remains on the map as grayed out pending reconstruction of the World Trade Center area.
The station finally re-opened as WTC Cortlandt St. in 2018, with 1 Train service, and is accessible via an underpass from the WTC Transportation Hub.