Why is "primer" pronounced with a short "i" sound?
Personal experience: It's not. Primer is pronounced with a long i sound, like miner or buyer, and I've never heard it ever pronounced differently.
Linguistic answer: Dictionaries vary on which is the correct pronunciation, but the OED, which is generally considered the final word on the English language, accepts both pronunciations as valid. The "long i" pronunciation occurs in etymologically related words such as primary and prime.
The short i pronunciation doesn't seem to be predominant (at least not in Southern California), so that's why we stick with the single "m".
The short 'i' tends to be used in American English, referring to the introductory textbooks. I have heard it quite often from good quality US media outlets (NPR etc.) so would assume it is regarded as standard. The British English is pronounced with a long 'i' (as in miner).
For the meaning of 'primer' as in a first layer of bonding material on a wall etc. before it is is painted, or in explosives/ammunition, the British pronunciation is, again, with the long 'i' (as in miner) and the American pronunciation is usually, in my experience anyhow, also with the long 'i' (as in miner).
The OED pronunciation of primer, n.1 is
Brit.
/ˈprʌɪmə/,/ˈprɪmə/, U.S./ˈprɪmər/,/ˈpraɪmər/
(in sense 2d) Brit./ˈprɪmə/, U.S./ˈprɪmər/, N.Z./ˈprɪmə/
That shows that apart from sense 2d, the ‘long i’ version is preferred in Britain and the ‘short i’ version is preferred in the U.S., and that apparently everyone says the ‘short i’ version for sense 2d (see below).
It also notes that:
Pronunciation with ‘short’ i (
/ɪ/) is original (and is still usual in senses relating to type); pronunciation (in the other senses) with ‘long’ i (now/ʌɪ/) seems to be first recorded in British dictionaries of the late 19th cent. and is the primary one given in all editions of D. Jones Eng. Pronouncing Dict.
Which says that the ‘long i’ version is new, and that the original and historic pronunciation is the ‘short i’ version, which America has retained more than Britain has.
The referenced sense 2d is
Chiefly N.Z. A class covering one of the first years of instruction in a primary school; a child in a primary school class.
Citation:
primer, n.1
Third edition, June 2007; online version March 2012. http://www.oed.com/view/Entry/151307; accessed 08 May 2012. An entry for this word was first included in New English Dictionary, 1908.