The purpose of subtitles is to generally allow audience members to mentally pretend that the words they're hearing are actually in their own language. In most cases, the purpose of intertitles is to allow the audience to pretend that they can hear things that they see the actors saying. In order for the latter mental substitution to work, however, the audience needs to be able to actually watch the actors, which means they can't be trying to read the text at the same time.
Using subtitles would have been a technical annoyance but not an insurmountable one, especially if one was willing to reserve space on the screen for them. Multiple-exposure photography was not difficult, and if one were using interpositives one could produce an internegative with subtitltes in different languages without requiring extra steps in the final printing. Handling multiple languages while using direct printing off camera negatives would have required more complicated printing steps, but
I think the much bigger issue is that even if subtitles had posed zero
extra technical difficulty, intertitles would still generally work better for dramatic purposes in silent films.