In the last decade or more speakers of American English have almost dropped using the word very as a modifier for emphasis or intensification and have resorted to the near-ubiquitous use of incredible/incredibly, in the face of the literal meaning of this adjective/adverb (‘that which cannot be believed’). The only explanation that suggests itself for this strange phenomenon is that of greater length of the word at stake by comparison with very. Quantitative increase is one way of iconically signifying semantic force, just as elongating the stressed syllable of any word (as of very itself) necessarily adds emphasis to it over and above the normal length of the vowel.
[ADDENDUM: As a couple of readers have pointed out, the use of incredibly can also be classed as HYPERBOLE. This fits one of the overarching themes of contemporary American usage, namely HYPERTROPHY, which has been instanced many times in earlier posts.]