In short, at the time the 757 line was shut down, the demand for its capabilities hadn't developed yet. Even today, I question if there is sufficient demand for such capabilities (in excess of that provided by existing narrowbodies) to justify an all new aircraft.
Time have indeed changed a lot since 757 went out of production. Traffic more than doubled and it seems every airlines is taking / considering 200+ seat A321 versions.
Mostly for short-medium <5hr flights. Apart from that, it slowly seems to sink in that the MAX isn't doing fine for the rest of the decade & something lean /modern is needed..
The MoM/NMA concepts have been shrinking since it's inception. Naming it a "Boeing 737 Replacement" is still a no go though.