Why haven't larger companies like Merck, Millipore attempted to enter this market? If the technology hasn't changed for 20 years and companies like GE have a huge market share, why hasn't a competitor tried to enter this space?

They have attempted, perhaps without success. There were many patented aspects when it comes to chromatography resins. Millipore Sigma does have some resins similar to Thermo Fisher. They can compete to some extent, but Thermo Fisher has products that Cytiva cannot compete with, despite their offerings. The credibility within the customer base also plays a significant role.