There's no answer to "why."

Although in my experience it's rare, in the United States people sometimes do pronounce the second "o" in chocolate (as a schwa). As a matter of fact, the American Heritage Dictionary online is showing me:

chôkə-lĭt, chôklĭt, chŏk-

so they apparently think chôkə-lĭt is the most common pronunciation.

In an entire 30-minute History Channel program about Milton Hershey, founder of Hershey's Chocolate, the narrator pronounces it as chôkə-lĭt throughout. One occurrence is at about 1:45.

http://www.biography.com/video/milton-hershey-full-episode-2074723193

Furthermore, in books--especially those written before 1950 or thereabouts--you will sometimes see the word spelled as "choklit" or something similar in quoted dialog, showing that the author believed this was non-standard or ignorant pronunciation--the correct pronunciation doubtless being chôkə-lit. A 1915 novel, for example, as a child saying:

"I don't s'pose he did ezackly," the child confessed. "But I was so afraid no one would want to paste pictures bad enough to come out today, that I promised 'freshments for all and a prize for the one who made the best book and Evelyn's got it. Evelyn, you better open up the box and treat the rest of us. A choc'lit drop would taste pretty good after working so hard.

I am almost sure that pronunciations such as "pleez" (for please), "choc'lit" (for chocolate), "veg'table" (for vegetable) were once considered ignorant, incorrect pronunciations that have now become standard.

Hi Owe, In both words, the schwa sound can be uttered but can also be dropped (in the ways you indicated). Schwa sounds never carry stress and so they are vulnerable to being dropped altogether in some words, especially longer ones. There are no rules I know of.  You just have to listen and repeat.

Probably Americans don't say "comftable" because they generally sound their "r"s when they appear before other consonants.

In English we have a pronunciation that changed throughout the time based in an old and almost static orthography. Because of that there is a huge difference between pronunciation and orthography.

Regardint to the R sound, in British English this sound is not pronounced at the end of syllables. For example, the contraction "aren't" is pronounced like /ˈɑrənt/ in American English, but like /ɑːnt/ in British English.

In the US, the most common pronunciation of "comfortable" includes the R sound but has only three syllables: kumf--tər--bəl.

And, yet again, the American Heritage dictionary say I'm wrong--that's the least common of the three listed. The most common is four syllables, with the "R" pronounced; next, three syllables, without the R, the way Michael Chambers says the British say it. And the way I say it--three syllables, with the R, is the third listed.

com·fort·a·ble  (kŭmfər-tə-bəl, kŭmftə-bəl, kŭmftər-)