i don't know the exact reason, i can only speculate, but it's easier for me to say ինտերնետ, կոկա-կոլա, պիցցա than ինթերնեթ, քոքա-քոլա, փիցցա...
but as far as speculating... many of those foreign words don't reach armenian directly but through russian, and in russian there are no sounds փ, ք, թ.
another reason can be that many words, such as internet in this case, were used in armenian before, but not from english, but from french or russian, where it sounds տ and not թ.
another reason can be that many such words in english originated from other languages (french, latin, greek, etc..) where they sound with տ, պ, կ, such as pizza in this case, and in armenian they get their original pronounciations, even though they may become famous through english (america).
note that all armenian pronounciations here are in eastern armenian.