Is there some historical reasons or is there a specific reason behind it?
This question is connected to: Why on-shell vs. off-shell matters?
Answer
A particle is said to be on-shell if it satisfies the relativistic dispersion relation,
$$E^2 = p^2 +m^2$$
in units wherein $c=\hbar=1$. If you graph it, you obtain a parabolic surface for massive particles, and a cone for massless particles, like a photon. This is known as the mass shell, it is quite literally a shell or surface. The momentum of a real particle can be represented as a vector on the surface, hence the expression on shell. Virtual particles do not have these on the surface, hence they are off shell.
Source: Perimeter Institute, A Deeper Dive: On Shell and Off Shell
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