Why linguistically the word of the city of Puyallup is difficult for non Seattleites to pronounce? It only contains sounds found in English.
It is locally pronounced as [pjʊˈæləp]. I don't actually know how it's mispronounced: I use [ˈpjʊwjəlʊwp] as a joking mispronunciation. Wiki says that an alternative pronunciation is [pjʊˈɔləp], which I could imagine but I've never heard. There is a certain element of randomness to orthographic vowel pronunciation, especially with native names which are for 99.99% of pastəds meaningless string of letters. Unless you're native, you stand a good chance of mispronouncing Yakima (should be [ˈjækəˌmɑ]). The sequence <all> is usually pronounced [ɔl], so it is predictable that it would be so pronounced by people who don't know better – the standard pronunication with [æ] is what's hard to explain (I suspect that spelling with <ll> rather than <l> is responsible). As Sumelic notes, orthographic all is usually [æl] when morpheme-medial (wallow being a minority pattern), and a spelling al would encourage the worse pronunciation [ɛ] (as in "Chehalis" [ʃəhɛjləs]). [ɔl] is still possible, if you ignore the morpheme-position influence on how <all> is pronounced. Interestingly, [ɔ] is closer to the vowel of Spuyaləpabš Lushootseed. (Wiki is just wrong about the etymology: it means "people of the bend", referring to the bends in the river).
The loss of <y> in the middle doesn't correspond to any general rule, so when people see a consonant in the spelling, they want to pronounce it. The dissimilative loss of [j] is one of those rare cases where difficulty of prounciation actually plays a rule on language change. It is articulatorily challenging to form, deform, and re-form [j] within less that 100 msc so the palatal gesture for the second [j] got attenuated.
Initial stress is possible, just not right for this word. There is is actual variation in pronouncing Swinomish (1st or second syllable), and you find second-syllable stress in non-native pronunciations of Yakima.