The origins of Adûnayân and the possible reasons for its unique character.
What are the sources for the language?
Here are the currently determined likely sources of various words:
Scholarly and scientific words from Quenya, occupying the position of English's Greek terms and some Latin. Quenya was the language of lore.
Legal, military and general "educated" words from Sindarin, occupying the position of English's Latin terms and some French. The upper classes spoke Sindarin for a long time.
Metalworking terms: pre-Beleriand Khuzdul borrowings for basics, Sindarin for Elvish techniques, Quenya for the highest Noldorin technology. It is difficult to believe that the Dwarves were still using Khuzdul around Men by the time the Edain arrived in Beleriand, or else there would be a greater influence upon the language.
Animal and plant names: pre-Beleriand Avarin and Silvan borrowings, plus any purely Mannish ingredient
Agriculture: Initial influence possibly from Silvan and Sindarin, otherwise most of it may be due to a probable Entwife origin for Edainic agricultural words, both before they crossed the Blue Mountains in the First Age, as well as Entwives met on the journey from Hildórien in the far east.
How did the language came to be?
Here matters become fuzzy. The Númenóreans were the result of the comingling of the three Edainic tribes that came over into Beleriand, each of whom had their own language. However, the language of the people of Bëor essentially became lost as they unanimously adopted Sindarin, while the house of Haleth was nearly wiped out completely by the end of the era of the Silmarillion, leaving only the house of Hador as a major influence.
Even then, it appears to have become (at best) a basis for constructing Adûnayân, for the language (at least is primitive form) bear heavy signs of strict and rigid codification of grammar. This strictness, along with its simplicity and the simplicity of its words, indicate a need to insure continuity of understanding with every Númenórean and a desire to preserve some of their heritage...on an island where there was great respect and usage for the elven languages.
Even when Adûnayân became the lingua franca of Númenórean, it still retained its simplicity of construction, instead smoothing the sounds of the language to adopt a more graceful aspect to its feel.