the passenger trains are mostly full and standing, both ways, all day, every day. there are no spare train units to be had to lengthen services or provide more but that's moot because there are no train paths available to run more services, nor can longer trains be operated because of infrastructure limitations at many major stations, including Euston, Liverpool Lime Street and Manchester Piccadilly.
there are no spare train paths available on the busiest sections of the line, which means more freight trains cannot be operated. A delay of a few minutes to an existing freight train at (say) Stafford causes congestion to ripple out to the point that commuter trains as far away as Rugby are delayed by more than 15mn. (I live this one regularly.)
HS2 would remove most inter-city services from the equation, leaving the existing main line better able to cope with growing local, residential and freight services.