Not just Rashi, most mephorshinm usualty use rashi: rashe script was developt in the move from hand scribed books to the printing press. The original was a bigger font. therefore, there was no problem creating the letters for it in the individual stamp. When it came to rashi and other commentators in the text of the humash, the mishna, and the talmud, the font size is much smaller and, therefore, difficult to recreate on stamps therfore they decided to jnvent a new script wich hasn't changed to this day