Consistency in quality and other metrics is the key, I think.

People like McDonald's because it's filled with sugars, salts and fats but also because they know what they're getting and it's (mostly) consistent from visit to visit.

I think Tom was also one of the earlier "cool/casual science & technology" channels and was probably actually one of the main drivers of this type of content. The only downside is that as other people have mentioned in comments, this type of content has gotten more popular, which seems great at first but it ends up being treated as entertainment rather than provoking thought and really being absorbed (not Tom's fault, just the way the average person consumes).