Blame the French. Arkansas is of French origin (so the trailing s is silent) but Kansas is English (so the trailing s is pronounced). Similar spellings of similar tribes, but the French wrote down the name of one, and the English wrote down the name of the other, and here we are.

http://www.slate.com/blogs/lexicon_valley/2014/08/27/why_the_state_names_of_arkansas_and_kansas_are_pronounced_differently.html

I'm not an expert, but in the course of answering what I thought would be a simple question I dug up both primary and secondary sources from earlier eras, including a rather long discussion by the Arkansas Historical Society circa 1880 (preserved in a publication from 1908) concerned entirely with the pronunciation of the state's name.

It appears that, in 1880, Arkansas' name was pronounced as it is today. The authors of the pamphlet launch into a rather amusing diatribe against people who pronounce it incorrectly (as in ar-KAN-sas with the final 's' pronouneced). Individual letters from committee members to the chairman follow, which are actually rather interesting and a very good read. They contain:

• Placing blame for the incorrect pronunciation on uneducated people, the damn Yankees, or haughty upper-class types who make stuff up, depending on the letter's author;
• A claim that, contrary to highest upvoted comment, "Arkansas" and "Kansas" in fact have different etymologies and were originally pronounced differently - one was supposedly written as "Accançeas", while the other as "Chanzes" - the claims are sourced from Joutel's Historical Journal of Monsieur De La Salle's Last Voyage To Discover the River Mississippi, published in 1687 (linked microform from 1846). Quick searches reveal that "Accançeas" and "Chanzes" are both mentioned as separate entitites, but skimming over the context, we'd need an expert on the 17th century French explorations of America to figure out whether "Accançeas" and "Chanzes" might refer to Arkansas and Kansas respectively;
• An early prescriptivist-versus-descriptivist fight, between President Noah Porter of Yale College and Leo Baier, President of St. John's College;
• The 19th century version of respect for the original inhabitants of the land;
• Henry Wadsworth Longfellow's opinion on the matter (he preferred today's pronunciation).

Here's my post below.

edit: oops, forgot a link

I'm sorry, but English is not my first languange and I don't quite catch what you're saying. What's the difference in pronounciation if you don't mind explaining?

As a reminder, please consult our rules about anecdotal answers. They are simply not fit for an /r/Askhistorians answer. There have been too many of these posted in this thread already and will be continued to be removed.

From a Kansas perspective: The name Arkansas derives from the same root as the name for the state of Kansas. The Kansa tribe of Native Americans are closely associated with the Sioux tribes of the Great Plains. The word "Arkansas" itself is a French pronunciation ("Arcansas") of a Quapaw (a related "Kaw" tribe) word, akakaze, meaning "land of downriver people" or the Sioux wordakakaze meaning "people of the south wind".

Pronounced differently for historical/regional reasons.

Tldr; Kansas was the Sioux word for "South wind people", and Arkansas is "down river people".

http://en.m.wikipedia.org/wiki/Arkansas#Etymology

From: http://www.statesymbolsusa.org/Kansas/KansasNameOrigin.html