Maybe it is that producers that hang out in the same musical corner of techno (for instance the funky Italians or the Drumcode-styled producers) use the same sounds and samples over and over again... but all in all I think that techno in itself represents a great variety in sound and texture.

You've got the loop-based tracks and you've got the story-telling records.
Both very different in style.

But I guess, if all producers would be more enthousiastic in their search for new sounds, it would evolve more.

Although I think everyone is very busy doing that... I don't think techno-producers are lazy bums. Some are just somewhat more talented, and have ideas how to innovate.
And me, as a DJ, must choose to use those innovative sounds.