Because it was deemed to contain the best information and was a generation on fro, the 1851. When it was released in 1982 the Mormons bought the film and the rights to trabscribe it, othe returns used street index and some ed's were surname indexed, but the Mormons had the computer software and expertise to do a full index which they then made available to search for free. By the time the 1891 came out in 1992, (I remember queuing at the old PRO to visit the catacoombs to view it) the FFHS had already decided to launch a country wide indexing project using it;'s members as volunteer transcibers , as all local reference libraries obtained a filmed version.
So the 1881 project itsself became an essentail research tool that bridged the gap from the GRO to the earlier returns
Did they have a blank Q17 back then to confuse East Anglians? And in Welsh for those in Wales, the level of language finishing "None of the above" written in the language (as in understanding it)?
I don't know as we only get to see the enumertors books, not the actual form that was delivered to each household, except for the 1911 return, we do get to see out ancestors opwn handwriting on that one, and how long they have been married and how many children living or dead they have had, imagine the upset a question like that would cause now, and you're worrried about blank questions lol