Just a hypothesis, but the core technology on the ISS is over 20 years old at this point, and I'm sure we have learned a lot ways in which we can improve upon the basic design and systems. There are standards used in the ISS design that any incremental update (think replacing one module at a time) would have to be backwards compatible with as it transitions, thus limiting any major overhauls. I imagine it would be more cost effective and allow for more improvements to just start from scratch using everything we've learned and create new standards