Real money is involved with subscriptions, unlike followers. The API gives details such as the total number of subs, what sub tier each of them is on and when each entry was created. Not all streamers (or Twitch) want everyone to have access to that information. Some streamers may be willing to talk about the sub count, or numbers of subs at the various tiers, but some (from my experience, most) streamers will want to keep that information private, or limit access to it to only the apps that they choose to give access to.
If you have a legitimate need to access subscriber information of a streamer, ask the streamer. If they say no then you’re out of luck, you have no business accessing those details.