I don't know the answer for definite but my assumption is because of the nature of modern websites. Most websites are not static, but rather complex mixes of static and dynamic content including various scripts running on the page. It seems like that would be complicated to duplicate, particularly without breaking something. A script running could easily break and throw unexpected errors if it's not duplicated seamlessly in memory.

I guess also as a result if Firefox duplicated everything perfectly from active memory, the onus is on Firefox to fix it if something breaks on some random website. If Firefox reloads the page in the duplicated tab, then the onus is on the website if something doesn't work in duplication. It's a fundamentally simpler technique and more replicable every time.

However as I say I don't know for definite. I do find Firefox does a reasonably good job of duplicating tabs in the current set up; I've not been aware of significant issues with websites and for me at least it's the starting from the same sort of place and having the history available that is most useful.