This is a highly speculative answer; I apologize.
In the Massachusetts Bay Colony in the 17th century, there were twenty one elected commissioners. (In the modern US, counties have only one county commissioner and several other elected posts, which are different depending on the city or county). One was an elected position (for example, regulating and judging proper fences and "salt pieters..."), so I think it's pretty equivalent to a register of deeds. Some duties for a "register of deeds" would have been held by a different commissioner. It doesn't make much sense for a commissioner to be electing another traditional "commissioner" position.
This position was held by lawyers typically, so also by using elections, they were choosing the person most knowledgeable about property law in the town. Today there are more opportunities for formal education, so no one would think it possible a lawyer could be self taught. Lawyers must pass the bar and be registered. Keep in mind though, that it isn't a requirement that a judge for the Supreme Court even be a lawyer!
Deeds, fences and other property disputes are the subject of many lawsuits between neighbors. A commissioner in the Mass Bay Colony could be called to court to testify. Everyone knew each other in the villages of the colony, such as Salem, so being elected meant more than just not taking bribes -- it meant being trusted to be objective in testimony to your friends and to the town's social pariahs. Objectivity and trust is why some judges are elected for instance. My opinion is that "register of the deeds" may be an elected position due to the always-contentious nature of property disputes.