When the .30 Herrett was introduced handgun hunting and silhouettte were in their infancy, and the developers (Milek and Herrett) didn't have a good selection of powders and bullets which would work. The little .30 worked better in the short barrels of the time than the .30-30 did (although today the .30-30 will give comparable performance to the Herrett). Forming brass was time consuming, but more important it was usually done incorrectly and the result was short brass life and mediocre accuracy for many/most .30 Herrett shooters. The word got out that the Herrett was tough to load for; combined with newer cartridges and longer barrels the old .30 fell from favor.
I still have two .30 Herrett barrels, a 10"(scoped for hunting) and a 14" (irons for silhouette), and they both still shoot very well. The .30 Herrett is an even better cartridge today than it was 30 years ago, because we now have more/better powders and much better bullets. In the 70s and early 80s the only bullets we could count on to expand in deer were the various 110-grain RN Carbine bullets with their poor BCs. Heavier 125s and 130s didn't expand well at all, except for the 125 Sierra FNHP, which had the BC of a brick. Today we can use the 110-grain Hornady V-Max, several 125/130-grain pointed handgun bullets, and the 125-grain Ballistic Tip. All work quite well at Herrett velocities. Today the .300 Whisper is a popular replacement for the .30 Herrett in short barrels, although the Whisper is several hundred fps slower than the oldtimer.