To answer the OP's question, the simple answer is supply and demand. People don't want them as much as they used to. This is probably due to the fact that as time goes on, autos are getting better. Some autos now get better MPG and perform better than their manual counterparts, which was unheard of back in the day. That, and many autos have a "manual" shift mode.
It does seem that the people (like myself) who like to shift like to do so out of nostalgia. My 2018 Ford Transit Connect was not available with a manual trans, or I would have gotten one. In the UK, they have an optional diesel engine w/ a manual trans, but those options are not available stateside.
Why not? Supply and demand. (edited)