Because it became a viral news story with London's Tier4 lockdowns plus France's initial ban. As soon as france did the ban, copy cats from around the world found out about it. You really think the ministry of San Salvador is looking closely at UK's new variant and decided to ban? No, they clearly just saw some news story, saw that others in Europe were banning so said "yeah sure, we'll do it also". I'm quite sure that in the next 3-4 days as this news becomes more viral almost every country in the world will ban travel from UK.