Without double checking (probably a mistake on my part lol), I think OTM butterflies using Calls are Bullish, OTM Butterfiles using puts are bearish, etc. So that is the reason. Neutral butterflies are created by using ATM options. ITM would be used if you wanted to make a bearish one with Calls or Bullish with puts, etc.
Stock at $50
If you expect the stock to go to $60, you would use OTM Calls - if you used ATM calls and the stock goes to $60, you would lose the investment.
JJacksET4