Why are all acronyms accented on the last syllable?

When saying acronyms out loud, almost always the last syllable is accented (no matter how long the acronym is): US*A*, U*N*, RSV*P*, etc.

Accenting any syllable but the last makes you sound silly (try it). Why is this the case?

One way of interpreting this is that in their underlying form, acronyms have stress on all of the syllables, but the rightmost one is what "wins out" when they are pronounced. According to some theories of English stress, it is a general principle that the rightmost "major stress" in a word receives the primary stress (or the accent). In these theories, words like "facilitate" are analyzed as having either no stress, or only a "minor" stress on the last syllable.

There is some evidence for the presence of underlying lexical stress on non-final syllables of acronyms. When a two-syllable acronym comes immediately before a word that starts with a stressed syllable, the acronym may be accented on its first syllable instead of on its second. E.g. even if "UN" is accented on the second syllable in isolation, it might be accented on the first syllable in a phrase like "UN peacekeeping troops".

Numbers ending in "-teen", such as nineteen, are a well known example of words that may show stress shift and that can be analyzed as having adjacent lexically stressed syllables.

The way I hear these and pronounce them myself, I'd surmise that the accent is an end-marker.

I don't agree with your observation.

I have heard not too infrequently,

• Could you please kindly R S ^V P my invitation to your own party?

  A mother's anxious request to her daughter for her daughter's engagement party.

• The ^U S of A today has won another gold medal.

  An excited news anchor.

• The ^U N has once again failed to take resolute action towards Syria.

  A person resigned to disappointment with the situation in Syria.

• ^U S - normally the first letter is accented.

• ^I B M, because saying I B ^M would be awkward.

• ^U K is more frequently expressed than U ^K. U ^K sounds awkward.

• ^DD T. D D ^T sounds awkward.

• ^F B I

• ^F U

Therefore, your observation is not accurate.