why French is Romansh with almost no Frankish elements?
I assume you mean Romance, as opposed to Romansh which is a related language spoken in parts of the South-East of Switzerland. I would however dispute the fact that French has almost no Germanic influence. Germanic languages had a great influence on both the grammar, the phonology and the vocabulary of French. See this page for more info.
You may want to consider asking your question in /r/asklinguistics as well as here.
I'm sure someone will be able to provide a much better answer, but just in case not, I will provide my limited understanding.
Germanic languages fall in three categories: North Germanic (like Danish and Swedish), East Germanic (Burgundian, Gothic, Vandalic, and Crimean), and West Germanic (Frankish, Saxon, Anglo-Saxon [Old English]. East Germanic was largely absorbed by Latin speaking populations, quite likely due to their relatively early adoption of Christianity and its Latin leanings. West German groups had less interaction with Latin speaking groups and migrated to England in several waves. Wikipedia (I know, not very scholarly) says that Viking incursions into England and the linguistic interaction with Anglo-Saxon may have lead to the transition of Old English to Middle English.
In short: North Germanic and West Germanic outlived East Germanic groups that adopted Latin based languages. North Germans and West Germans then had greater interaction and influence with England.
FYI, there's a section in the FAQ on the first question
edit: Oh, I had another thought: there's also a section in the FAQ on the transition from Frankish to French. As I recall, questions/responses are more about the when & how, rather than why, but it may be of interest in setting context, and may(?) at least indirectly address why.