Why in the mid nineteenth century of all times was there seen to be an overhaul of the provisions which surely existed in other forms previously barring such age old offences as assault/rape/murder/etc? What brought about OATPA1861?
In simple terms it was part of a wider effort to consolidate and simplify criminal law referred to as the Criminal Law Consolidation Acts 1861 (which were themselves revisions of the Peel Acts)
Basically the sheer number of acts was causing administrative headaches - the courts and parliament were losing track of what was and wasn't in force. Since at least nominally ordinary English folk were supposed to know the law it was getting a bit ridiculous that the people whose literal job it was to make and adjudicate these laws weren't able to!
It would be superfluous to attempt to enumerate the causes which have led to the passing of these statutes : the circumstances which led to their occupying the prominent attention they have received were twofold: ist. The Court of Queen's Bench, a few years since, spent some time in solemnly considering the effect of an act which was afterwards found to have been repealed some years previously, but of which repeal both judges, counsel, solicitors, and all the parties concerned were, at the time, profoundly ignorant, and 2nd, Very nearly at the same period an act was formally passed by Parliament to repeal several acts which had been actually repealed about 20 years previously. As it is a legal axiom that every Englishman is presumed to know the laws of his country, and as the preposterous proceedings just mentioned cast a grave suspicion before the eyes of the public upon the accuracy and extent of the knowledge even of judges and legislators, it was felt that some means ought to be adopted to determine with clearness and precision what laws were really in force and what were abrogated : hence arose the question of “Statute Law Reform,” which for many years has received the intermittent attention of the legislature, and been handled by commissions of inquiry almost innumerable and uniformly expensive.
PS: If you're interested in the general topic of how these acts came to be and the legal environment at the time it's well worth reading the whole "Preface" section of the book I linked to above.