This is because we are translating the name of ISIS/ISIL/DAESH from Arabic.

ISIS used the term Al Sham which is a region name from hundreds of years ago unfamiliar to Americans. The name translated to The Islamic State of Iraq and al-Sham

Bilad al-Sham comprised the area of Greater Syria, spanning the modern countries of Syria, Lebanon, Israel, Jordan, and Palestine, as well as the regions of Hatay, Gaziantep, and Diyarbakir in modern Turkey.

This region is also called the Levant. Kinda. It's not exactly the same regions, but it's a word at least more familiar with Americans. If you ever read/seen The Maltese Falcon, (great book and movie, btw) the character played by Peter Lorre was described as a Levantine.

So this where the L is ISIL comes from. Congress and the Military used ISIL for a while too. News organization went with ISIS which is kinda easier to say.

BTW, the CIA still says Usama Bin Ladin, because that's is how they first spelled in their files https://www.cia.gov/library/abbottabad-compound/index.html

Wow, looking this up I didn't even know this was a controversy....apparently Trump attacked Obama for this in 2016 and then in 2017 his White House used ISIL in a report in under reported terrorist attacks.