Twitter's technical follow limits: Every account can follow 5,000 users total. Once you've followed 5,000 users, there are limits to the number of additional users you can follow. This number is different for each account and is based on your ratio of followers to following; this ratio is not published.
I'm looking at the phrase "This number is different for each account" which seems to answer your question.
Both this and the daily follow limit are likely set to prevent abuse of Twitter. Those numbers would need to be adjusted over time to prevent abuse of Twitter.
Behavior outside the norms in a global scale social media like attempting to follow every account would quickly make Twitter undesirable for the targets of such campaigns. To prevent that level of abuse limits would be enacted. For accounts that have legitimately grown and for which people show interest the limits are likely raised to allow the famous or topical to avoid those limits. Sorting out who is an abuser and who is famous or topical is going to change over time, but as indicated above one of the things that they are looking at is the ratio between followers and following.