[Ymani Cricket]

There's a number of reasons. Peole started using #obamgate instead because of this rule (and other rules they apply):

“The Trends list is designed to help people discover the ‘most breaking’ breaking news from across the world, in real-time. The Trends list captures the hottest emerging topics, not just what’s most popular. Put another way, Twitter favors novelty over popularity…”

[bagster]

“The Trends list is designed to help people discover the ‘most breaking’ breaking news from across the world, in real-time.

Okay, I get it now.

A former President of the United States being exposed as having lead a takedown of his successor using the mechanisms of weaponized government agencies and breaking every law in the book isn't BREAKING NEWS.

Makes perfect sense.

#YayTwitter

[Yulee]

Put another way, Twitter favors novelty over popularity…”
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Then rename it HeadRatgate.