Why does the Sudan have a "the" in the name?

I am endlessly perplexed why the country Sudan has a the in the name: "I am from the Sudan". Why not "I am from the Kansas" or "the China"?

I understand the the in "the United States" because a non-personal noun "states" is part of the name. But why "the Sudan"?

This is a nice explanation from a professor of etymology, via the BBC:

Professor Liberman says the habit of putting "the" in front of place names is heard throughout the English-speaking world and is common to Germanic and Romance languages.

"In general, use of the definite article is unpredictable. Why should it be London but The Thames? There is no logic for it yet this is the way it is.

"Sometimes country names go back to river names. As late as the mid-20th Century, everyone said The Congo because Congo is the river and named after the river, but no-one says it any more."

He suspects that people once preferred to add the article if the place name related to a geographical feature like a group of islands (Bahamas) a river (Congo), a desert (Sudan) or mountain range (Lebanon).

"Later the phrases were shortened, but the article survived. Hence the arbitrary rule that river names, the names of deserts and mountain ranges need 'the'.

Technically today, it seems Sudan is not the same thing as the Sudan.

Sudan is the region:

… Sudan is the name given to a geographic region to the south of the Sahara, stretching from Western to Eastern Africa. The name derives from the Arabic bilād as-sūdān (بلاد السودان) or "land of the Blacks" (an expression denoting West and Northern-Central Africa). emphasis mine.]

The Sudan is the country:

The phrase "The Sudan" is also used to refer specifically to the modern-day country of Sudan, the western part of which forms part of the larger region, and from which South Sudan gained its independence in 2011.
(The) Sudan (Arabic: السودان‎ as-Sūdān Listeni/suːˈdæn/ or /suːˈdɑːn/;[9]), officially the Republic of the Sudan (Arabic: جمهورية السودان‎ Jumhūrīyat as-Sūdān) and sometimes called North Sudan, is a country in North Africa, … [emphasis mine.]

'Sudan' without the definite article can also mean various other interesting things, including (a nickname for) Dallas, Texas, in the United States!

"The" in "The Sudan" is used simply because it transliterates from the Arabic "As-soudaan".