What are two major reasons the Navajo language made the perfect uncrackable code for the Allies during WWII?

Outside of the Navajo peoples, the complex and largely unwritten Navajo language was unknown. You can't decode what you don't know.

Navajo did not have military terminology. The initial 211 terms (later expanded to 411) were new Navajo terms with military meanings.

The resulting code was fast, secure and succinct. It proved unbreakable.

https://www.azcentral.com/story/news/local/arizona/2018/07/11/navajo-code-talker-facts-unbreakable-code/460262002/

Some background to this fascinating unbreakable code:

NAVAJO NATION - INVENTORS OF THE UNBREAKABLE CODE

Amongst the most urgent challenges the United States faced in World War II was the capability to send, receive, and decipher codes - preserving U.S. military secrets against foreign adversaries. This capability would give the nation the edge it needed in the battlefield. This was not a new challenge for the United States. During the First World War, the U.S. employed hundreds of Native Americans - Choctaw, Comanche, and Cherokee - to safely and securely send code overland and seas. Following the war, other countries dispatched people to the U.S. to learn these languages and effective methods. Philip Johnston, a World War I veteran who grew up on a Navajo reservation in Arizona, recognized the critical importance that linguists played in the previous war and how the Navajo language could provide the secret sauce for U.S. code-making.

What makes the Navajo language unique? It is not written down.

The spoken word would provide an extra layer of complexity for enemies attempting to intercept U.S. secrets. Seeing the promise of the Navajo language, he advocated for their service to the Marine Corps. Almost immediately, the Marine Corps green-lighted the proposal and their enlistment. Three months later, 29 Navajos arrived at the Recruit Depot in San Diego to begin basic training. They were asked to join a special program being set up along with other Navajo-speakers who had already enlisted in the Marines. After completing basic training, the Navajos moved to Camp Elliot in San Diego, where they received special training in radio transmission and operation and, together, devised the first Navajo code.

The code was extremely complex and had to be fully memorized by each code talker. It consisted of 211 Navajo words that were then given military meaning. For example, "fighter plane" became "hummingbird" ("da-ha-tih-hi" in Navajo), and "submarine" became "iron fish" ("besh-lo" in Navajo). The code was an intricate web of words designated to military terms and individual letters - it was impossible to break. On August 7, 1942, the Navajo Code Talkers conducted their first major operation - the 1st Marine Division, along with 15 Navajo Code Talkers, hit the beaches of Guadalcanal. This was the first occasion where Navajo code was used in battle - and with success.

From 1942 to the end of the war in the Pacific, Navajo code talkers were used in every major operation that involved the Marines. In one famous example, during the nearly month-long battle for Iwo Jima, six Navajo Code Talker Marines successfully transmitted more than 800 messages without error. The number of Navajos serving the U.S. war efforts would grow to more than 375 over the course of the war - contributing to the Guadalcanal Campaign, the battles of Iwo Jima, Okinawa, and Guam, among many others. The Japanese military, once exceptionally adept at intercepting and decrypting U.S. code, was unable to decrypt a single word from the code talker.

https://www.intel.gov/publics-daily-brief/public-s-daily-brief-articles/1090-navajo-nation-inventors-of-the-unbreakable-code

The code-breakers were honoured 56 years after WW2 ended, in July 2001:

It is estimated that between 375 to 420 Navajos served as code talkers. The Navajo code talker program was highly classified throughout the war and remained so until 1968. Returning home on buses without parades or fanfare and sworn to secrecy about the existence of the code, the Navajo code talkers are only recently making their way into popular culture and mainstream American history. The "Honoring the Code Talkers Act," introduced by Senator Jeff Bingaman from New Mexico in April 2000, and signed into law December 21, 2000, called for the recognition of the Navajo code talkers. During a ceremony at the U.S. Capitol on July 26, 2001, the first 29 soldiers received the Congressional Gold Medal. The Congressional Silver Medal was presented to the remaining Navajos who later qualified to be code talkers. Senator Bingaman's legislation was one attempt to answer the question of how the United States should document and remember the Navajo code talkers.

https://www.archives.gov/education/lessons/code-talkers