In terms of options should you not get into the MD, pathology doesn't really have more per se than micro/immuno. You can always not do breadth in 3rd year and take subjects from both (or if in Biomedicine, do the Defense and Disease major if you're keen).
As for why it's more relevant, two reasons (this is my opinion). One is that pathology is almost universally taught badly because of the nature of the content. And two is that in the first year of the MD, there are far more people wishing they'd studied micro than path. You can, in theory, pick up pathology quickly if you understand the basics as per my comment earlier about getting the fundamentals down. I spend pathology practicals working with some friends rather than sticking solely with the prescribed notes because it works for us better that way.
In contrast, microbiology (and immunology) is a clusterfuck of specific information eg; know that E coli stains gram negative, is a rod, is a lactose fermenter and grows pink on MAC agar, is 80% of UTI infections etc. And then repeat for other organisms. And then throw the immune system in and it gets even worse. I'd rather have the knowledge base for the very specific information that just requires memorization than the conceptual stuff that requires a deeper understanding.