What an interesting question, I now know that only sugar and sure (along with sumac) are English words pronounced with an 'sh' !
It seems to concern pronunciation from the French to English (Arabic comes into it too). Here's a piece I found which I haven't got time to edit, hope it helps.
"The sound of French /u/ (a close front rounded vowel) is denoted [y]. Sugar is from French sucre [sykʀ(ə)], and sure is from French sur (e) [syːʀ].
Sugar, sure and sumac are the only three in Modern English, but historically there were others. In the sixteenth century a phonetic change of sy- to sh- was attested (in the shape of sh- misspellings) not just in the words sugar and sure, but also in words like suit (variously spelled shute, shutte, shuite and shuett), suet (spelled showitt, shewet and shuet) and sue (spelled shue). By the nineteenth century, the sy- pronunciations won out for all but sugar, sure and sumac (and was later replaced by plain old s-), though the sh- pronunciation also survived in the middle of some words like issue, tissue, assure, ensure, insure, pressure, etc."
From a website english.stackexchange.com/questions/33434/why-are-sugar-and-sure-pronounced-with-an-sh
