>I wonder why OED is opting to use the electronic meaning without giving the "real" SPAM a mention.

unwarranted conclusion! I gave OED's new entry for spamming, which stems from the verb 'to spam'†, which in turn stems from Hormel's SPAM™, of which they say [App. blend of SP(ICED ppl. a. + H)AM n.1, but see also quot. 1937]

1937 Squeal 1 July 1/2 In the last month Geo. A. Hormel & Co...launched the product Spam... The ‘think-up’ of the name [is] credited to Kenneth Daigneau, New York actor... Seems as if he had considered the word a good memorable trade-name for some time, had only waited for a product to attach it to.

† the verb has two senses:
1. to give a person an unpleasant task [rare, Brit. milit. slang]
2. (the computer slang) [influenced by MP usage of SPAM]

edit: the significance of the source of the 1937 citation just now barreled in..