It's not that the "strain" changes as much as the plant just gives out.
It's not a cannabis thing, it's a 100% proven and well documented phenomena called Cellular Senescence.
You see the same thing in mushrooms. You can only take production oyster mushrooms for 4-5 generations max before you have to collect spores, make cultures, and start again.
EVERY species goes through this process, it's just at different rates. You'll find "strong" strains of mushrooms that can get 6-7 viable generations or a "weak" strain that gives out after 1-2....
en.wikipedia.org