Why is TWICE written 트와이스 in Hangul, instead of 톼이스 ???

Because 트와이스 means TWICE. Why would it be the other?

I guess they preferred the sound of the first transliteration. Try to hear the difference here.

It sounds a bit more natural to their intended sound than the one you mentioned

It's due to Korean being a syllabic language so there are 4 syllables in TWICE, it's also a reason why Hangeul is made for Rapping

There’s not a single word in Korean that uses 톼, even though it is a technical possibility in Hangul. For all intents and purposes it is a non-existent letter combination.

Quote from GWSN-Puzzle

트와이스 is 4 syllables

톼이스 is 3 syllables

트와이스 is like teu-wice-eu .... but 톼이스 should be twice-eu (?), which is closer to the English pronunciation

Koreans aren't trying to copy English pronunciation but incorporate it into the sounds of their own language, this is normal for all languages that use languages that are not native to them. Fan is pronounced as Pan for Koreans.

Koreans would have to rewrite their own language to incorporate the sounds of other languages and that's largely why there are different accents for different cultures.

Quote from GWSN-Puzzle

트와이스 is like teu-wice-eu .... but 톼이스 should be twice-eu (?), which is (slightly) closer to the English pronunciation

the only difference is 와 which is used because "TWICE" ends in a vowel, i think but i cant remember

Quote from GWSN-Puzzle

Yes, but the 톼 (twa) syllable DOES exist (although there is NO f-sound)

Except it's not used for English words and it's simply not how Koreans would ever sound out English words.