Why do so many people pronounce "馄饨" as hún dùn?
Because native speakers don't actually know the tone without being taught it. 

Northern Mandarin pronounces it as húntun ~ húndun, with the second syllable having the light tone (轻声). The morpheme 饨 is not used anywhere else, so it only appears in native speech without a tone. 

It's not rare for native speakers to reinterpret this syllable as "originally having the 4th tone" because of the acoustic similarity when pronouncing it with the light tone (there's a slight phonetic fall particularly as it follows a 2nd tone syllable). So when they try to shake off the light tone accent which is associated with being regional vernacular, they arrive at húndùn.



Note: Pronouncing t as d is unusual. It may be a sporadic change. The only other word I can think of, where the initial (声母) may be lenited in a light tone, is 糊涂 hútu ~ húdu. Many people do "correct" the pronunciation incorrectly to húdù, but this problem here is less severe because, unlike 饨, 涂 is widely used elsewhere with the known pronunciation of tú. Hence when speakers (especially literate ones) try to enunciate it, they arrive at hútú correctly.