The reason is actually a combination of cost to replace all set top boxes, and the fact Verizon like the cable companies are losing tv customers hand over fist. Coupled with the fact Verizon wants to ditch copper lines (dsl) and concentrate only on Fios broadband and wireless. But at this article it states Verizon was testing 4K
https://www.nasdaq.com/article/verizon-teams-up-with-ses-4k-content-for-fios-tv-customers-cm766941
But tell yourself if your customer base was dwindling would you invest in 4K to just keep a 100,000 customers? Capital wise I would not care.