What is another word for but why?
Pronunciation: [bˌʌt wˈa͡ɪ] (IPA) 
When someone says "but why", it typically means they want to understand the reasons behind something. There are many synonyms you can use instead of "but why" to express this sentiment. Some alternatives include "what's the rationale?", "could you explain the reasoning?", "what's the motive?", "what led to this conclusion?", "what's the underpinning?", or "what's the logic behind this decision?". Depending on the situation, choosing the right synonym can help you communicate your thoughts more effectively and make sure you're asking the right questions to gain a deeper understanding of what's being discussed.
