Interesting; in a lot of articles on the history of roleplaying, Traveller is presented as the #2 game in popularity in the late 70s and early 80s, at least.
Let me put it another way... I started RPGing in 1981. Traveller was nowhere to be found, but D&D was in Longs Drugstore. As were DQ and several other 3rd tier games. Plus RQ (a 2nd tier game).
A few years later, and I was into Traveller (fall '83) - two stores in Anchorage carried Traveller.
Circa 85 -
Book cache carried TFT, and the full range of SPI and Metagaming microgames.
Longs carried DQ, RQ, D&D, Star Frontiers, 007, MSPE, and Top Secret.
Game Keeper had the full AD&D lineup... plus the full GDW lineup. Plus Palladium's lineup. And Space Opera, 007, and MSH.
Spenard Hobby had literally 30 cubic feet of RPG stuff... including FGU's full lineup.
Eagle River Hobbycraft carried Traveller and D&D; Dimond Hobbycraft carried full TSR and GW. Other anchorage locations carried only D&D.
Bosco's Comics carried only used games. I got into BRP that way, with a used copy of ElfQuest.
Of the lot, only Book Cache, Boscos and Dimond Center Hobbycraft are still in business. Book Cache carries no games, hasn't since the 90's. Boscos migrated into new games in 86... and being a block from Spenard Hobby, ran them out of business by a nickel a copy...
Further, B Dalton, JC Penny's, and Waldenbooks carried D&D. As did the Ft. Rich and EDF PX's...
I've met
many a gamer who never heard of Traveller until they saw/heard of it from me. Often, they came in with D&D, and went the palladium route. Or later on, came in via WWG.
Traveller became 3rd tier sometime around 1991... not by losing ground, but because the first tier was so wide in the markets, and a second tier not tied to game stores arose: WWG's VTM avoided many game stores. Palladium started selling to the book trade. GDW put the wrong products into the book trade...
RQ/BRP, T&T, Traveller, and WFRP all became 3rd tier... becuase a new tier opened up between D&D and them. Palladium made the leap... popular licenses (Robotech, TMNT) and innovative setting materials, and a simple system. (I don't like it, but it's playable.)