Maybe that's part of it. They were basically the face of thrash metal in the 80's but you have to remember that the rise Metallica was a slow burn. Sure, they had 3 albums before Testament released The Legacy, but those early Testament albums are really one dimensional to me, musicwise and vocally. I never considered Testament in Metallica's league, honestly.
Metallica's songwriting was so much better than your average thrash band. Hell, in some circles, Metallica stood so alone that they were even referred to as Alternative.