It's got a lot to do with history. Back in early 2000 TM was THE defacto AEG manufacturer. All others were basically hit or miss or the same amount or less money, most the time it was miss. There were lots of horror stories with mech boxes cracking or gears grinding right out of the box with guns not made by TM.
Classic Army was notorious for this with their first generation of AEGs, to the point of which the mainstream advice was buy a CA gun, throw out the mech box and put in a TM mech box and motor instead. That went on for a few years, and it essentially became doctrine to the point of which it was the unquestionable truth for a long while.
Then wonder of wonders, CA start tightening up their quality control, and other companies started coming with reasonably QCed guns for considerably less then what a TM would cost. Nowadays clone AEG companies are a dime a dozen, but there are quite a few older players that have long bitter memories struggling against first generation clones.
Nowadays CA and TM are not THAT much better then the clone companies, but they are slightly better out of the box due to stricter quality controls. It becomes moot with upgrades and tuning, and I'd still buy the occasional TM gun because of that. TM also is the forerunner of AEGs and some GBBs, they spend the money on R&D for new guns to come out with whilst the others save on that cost and produce clones for less.