There isn’t only one reason for that to happen. I have two old smartphones around. Both are no longer usable with any cell plan in Canada. The first (and oldest - originally mine before becoming a hand-me-down to a relative) went out of use after the elderly person who was using it had it active for a year or two but then stopped bothering with it due to his illness keeping him at home and preference for larger screens in general. The other I’d still be using (still use it for some things!) if I could. I have always used my computers, phones, etc. as long as I could get away with but for #2, I had to get a newer (but used) phone to keep service after the end of 3G in Canada. The new phone is definitely nicer in some ways but I wouldn’t have changed phones if I had a choice. 🤷🏾‍♀️