I currently do not have an HD TV. I pay the extra to get the shows I want. Now I am finding that when I go in to watch a show On Demand it will only be available in HD, not SD. Why is this happening? I never had this problem before. I also noticed that some TV shows are not available to watch on demand at all. For example Lucifer is only available to episode 9 even though this week was episode 12. I want to watch episode 12 before 13 comes on. Why is there this lag time now. Also noticed it with Outlander. For the longest time it was only available in HD. Outlander was the reason I got STARZ and then I couldn't even watch it.
SD is slowly being phased out. These days most people have HD TV's, they are no longer cost prohibitive like they used to be. Most people, myself included, no longer watch anything in SD. Here is an article on how DIRECTV is completely phasing out SD by 2019. Don't be surprised to see others follow suit.
I assume you just have a SD STB. If you spend a couple bucks more (or maybe none) and get a HD STB, it will still work with SD TV.