Until GURPS, and MSH in the early 80s, the RPG world was dominated by 4 games: D&D, Runequest, Traveler, and T&T.
Champions tried to make some inroads, as did V&V, but superheroes never fully caught on until TSR released the MSH game.
Top Secret sold a bunch, but good luck finding anyone who actually played it.
It was the same "big 4" until early 83, and the suddenly you had a huge explosion in options over the next 3-5 years or so, with GURPS, Star Frontiers, T2K, the first mass-retail editions of Chaosium's Cthulhu and Elric games, the FGU a-game-for-every-possible-oeuvre, SPI's attempts at Universe / DragonQuest, Indiana Jones, Gangbusters, Recon, Mechwarrior, the goofy one-book games from Tri-Tac, Rolemaster (which everyone seemed to own and no one played), Harn (which no one seemed to own but everyone claimed to have read), and the UK SJ's Fighting Fantasy gamebooks.
But at the outset, T&T was one of the very first "it's like D&D, but here's how it's different...." kinds of games, and Ken St Andre was always big on solo support as he realized that not everyone wanted to fly their geek flags too high if they wanted to survive high school.
T&T got (unfairly, IMHO) kicked to the curb as the D&D 2e explosion took off in the early 90s, as there wasn't a great "game world" built around it, and there wasn't really enough differentiation between it and BECMI D&D as a rules set. It was fun, but fell behind other more sophisticated "product lines" at a time when the RPG world was rapidly becoming more consumerized and moved out of the fan-driven first 10-15 years of the hobby.