For the same reason that "mano" is feminine!

Hello Patrick, 'A' in the noun 'idioma' doesn't turn it from a masuline noun 'idiomo' into a feminine one 'idioma' . It is not significant of a feminine being here. It is considered hence as any other letter. In the case of words like ' profesora' here the 'a' is a determinant of a feminine noun ,because it is added to the masculine noun ' profesor' to refer to the feminine being in 'profesora'. In the case of such nouns * there are plenty of them in the Spanish language* , the 'a' will indicate a feminine noun. In the case of 'idioma' and other nouns as well the 'a' is treated the same as any other letter or vowel and is not determining the masculinity or femininity of that specific noun. In the case of the noun 'luna' for example ,it is a feminine one , but not because it is ending in 'a' , here the letter 'a' is just a component letter of the word. There is no 'luno' and the feminine equivalent 'luna'. In general the gender rule in Spanish is that the nouns ending in 'a' are feminine, in 'o' are masculine ,but there are some that break this gender rule as for example : Ending in 'A' , yet masculine nouns: el aroma el sistema el sofá el teorema etc... You could check the following list for more Spanish words breaking this gender rule: http://spanish.about.com/cs/grammar/a/genderreversal.htm

There are 'general rules' which help you to learn spanish. However, it doesn't mean that they will always work! There are exceptions, and the word 'idioma' is one of them. It is 'el idioma'.