Why don't particles from surrounding interstellar medium show up in the graph as matching the pressure of the solar wind?

Presumably simply because there isn't as much density. The interstellar medium particles must be less dense but more energetic, thus producing the pressure that causes the heliosphere to be restricted.

As I understand it, the much smaller number of particles hitting Voyager now are the interstellar medium. The rate of particles hitting Voyager is not a measure of the pressure of the ambient plasma.

Correct:

The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance. The crossing of the heliopause should be signaled by a sharp drop in the temperature of solar wind-charged particles,[30] a change in the direction of the magnetic field, and an increase in the number of galactic cosmic rays.[34]

https://en.m.wikipedia.org/wiki/Heliosphere#Heliopause