Romantic words were incorporated differently into other languages as the Roman Empire's influence spread. Words like inflammable have more direct Latin roots than words like indebt. So where Inflammable comes directy from the latin inflammo, indebt came from old French and middle English, going from something like endetter and endetten, using the root word dete and dette, but stemming from the root Latin word, debitum. Then en became in at some point.

That's how I understood it anyway through a very short amount of google searching. Anyone feel free to correct me.