I can look up the etymology for indubitably... but how did a word like this, which pretty much exactly parallels "undoubtedly," come into use? This might be an unanswerable question, but how did two words with such parallel meanings, the only nuance distinguishing them being that you sound more pretentious if you say "indubitably", come to coexist?
There is actually a subtle distinction between the two words: undoubtedly means "not doubted" whereas indubitably means "beyond doubt" i.e. that which cannot be doubted as opposed that which is not doubted.
This answer serves to illustrate, in part, the more general question you've raised. There is a phenomenal plethora of synonyms in English (Hungarian takes the trophy, though) and it is largely due to the language's mongrel heritage, in turn caused by successive waves of conquerors and invaders putting down roots in the land. We have roots in a number of Celtic, West Germanic and Romance languages, and so an extraordinary arsenal of vocabulary has been available to us. As a result, we have been able to use these synonyms to inject the language with subtler shades of meaning than would be afforded by a single linguistic root. Why this quality is more apparent in English than in some other languages is not entirely clear (we're certainly not unique in our mongrel heritage) and the debate continues among linguists today.
Regarding "pretentious" synonyms, I believe this has its origins in the days when French was the language of the court and Latin that of the clergy, while common old guttural, germanic, dialectic English was the language of the peasantry. The association of Romance languages (and therefore longer words of latin-style inflection) with the higher classes of society has stuck with us, and the hangover of this can be observed, for example, in the extensive use of French in "legalese."
None of the above are more than simple explanations for a word in a language that has evolved over millenia. Perhaps the Oxford dictionary helps from an etymological point of view - thus from 'dubitabilis and dubitare - latin and French sources . ' Undoubtedly 'though ( ! ) the words have generally the same meaning - or similar meaning, the constructive elements in the two words are quite different. Indeed, how the two words are spoken seems to me to be poles apart. ' Doubt ' evolves from medievil English languuge but perhaps with the influence of the French old French word 'doter ' - saying doter ( with an accent sounds very much like ' doubter ' ! The two are very close in terms of sound. The source is the same for both of the latter. I venture that three cultures have designed the word over time. Essentially a lot of our languages are changed from one culture to the next . but by the way the new user sounds the word. As for the ' prententious ' use of the word indubitably I can only say that I believe this is a flawed interpretation defined from the subjective self - what is wrong with the idea of pure refinement of language and the manner and the sound in which it is spoken ?