What has proven to make Sartorius so strong in this space? Why haven't other companies been able to break in?

It's been somewhat of a blind spot. No one else moved in that direction before Sartorius, and now others are trying to enter the same space. It was a niche between the leads and the actual bioprocessing. It wasn't recognized until a few years ago, but now it's shown to be a very valuable area. When something is valuable, competition inevitably follows.