As I mentioned, independent computer stores—the Byte Shops, the Computerlands—didn't bother to carry TRS-80 products, despite the huge customer base, because they understandably didn't think they could compete against Radio Shacks everywhere. Same with toy and department stores that after 1980 became the primary home-computer sales channel for Commodore and Atari. Since company-owned Radio Shacks couldn't carry anything without the Tandy logo, 80 Micro and other magazines became the only way for third-party developers who didn't or couldn't get approval for resale by Radio Shack to reach customers. And since corporate policy prevented Radio Shack clerks from admitting that third-party magazines or products existed (even while a Tandy executive wrote a regular column for 80 Micro, and the company regularly advertised in its pages), the only way a TRS-80 or Color Computer customer knew of this gigantic ecosystem's existence is if a friend told him, or he happened to walk by a newsstand with 80 Micro or Rainbow magazine.
Compare this to Apple. A very important factor in the II's early popularity was school districts buying it to run educational software from MECC like Oregon Trail and Lemonade Stand. But this was not inevitable. A teacher or administrator in a rural school district in 1979 looking to purchase computers would naturally look to the Radio Shack in town, but would only have found incredibly crude Tandy-published software. Even with such handicaps Radio Shack had a substantial portion of the educational market, which after 1980 quickly eroded until 1985, when Tandy had an unexpected second computer boom driven by the PC-compatible Tandy 1000.