I’d argue that Smiler reopening was slightly different. Smiler was a major coaster that had been shut for less than a year, so it was still firmly ingrained in the public conscious. Its reopening was also unlikely to have entailed any significant CAPEX investment.
Sub-Terra, on the other hand, was a filler attraction that has been shut for 8 years and not even acknowledged by the park for a solid 5, so I wouldn’t mind betting that it would need re-selling to a good percentage of the park’s current guests to justify the investment into reopening it.
For most current Alton Towers visitors, Sub-Terra may as well be a brand new experience if it reopens. That wasn’t the case with things like Smiler and Hex, as these were only shut for a shorter period of time.