I can only speak for myself, as I have an interest in aviation history as a former airline pilot. I don't think it is necessary to have formal education in science to study the history of technology. In my view it suffices to grasp the essence of the research And more importantly, as historians do, to evaluate it's impact on society / use of the technology. Just like flying; you need some basic understanding of how an aircraft functions but there's no need for the pilot to be able to design an aircraft. Nor is there a need for an aircraft designer to be able to fly the thing themself.

Doesn't this thread need a Meta tag?

It's not uncommon for historians of science and technology to have technical backgrounds of varying degrees, though my intuitive feeling is that historians of science are more likely to have scientific background (aka, undergraduates or even graduate work in a science), while in historians of technology this is less the case. I don't think this says anything profound except that the "feeder" disciplines for historians of this sort probably vary a bit (I don't think too many engineers become historians, whereas there are a number of biologists, physicists, etc. who decide to become historians).

But anyway — it's not uncommon, no. How common is it? I don't know — I've never seen a survey of this sort before, and my limited anecdotal experience is, well, limited and anecdotal.

As for why it is or isn't: depending on the question you are asking about the past, a present-day education in the subject may or may not be useful. Sometimes it is. Sometimes it is not. A modern understanding of physics does not help you understand Hobbes' or Descartes' physics very well — they differ dramatically from our present-day understanding of the same topics they tackled. Even in studying modern topics, a technical background may not help one understand what is essential a question about social or government matters (e.g., if you were studying the funding of science in the 20th century, it would not really require a deep understanding of the technical matters). But of course if you ask certain types of questions, then a technical understanding is necessary, though that may not require a formal degree in the subject matter. For example, an advisor of mine wrote a social history of Feynman diagrams — the fact that he had a PhD in Physics along with a PhD in History made such a project feasible. I have written on the history of classification and laser fusion, and found that I could pick up the technical matters necessary to write such a thing with some effort, coupled with the fact that I am friends with several physicists who could help me understand any niggling technical points, like what "isentropic compression" meant in a certain context.

There are some histories of science/technology that are very technical, there are some that are very non-technical, there are some that bridge the gap. It is worth noting that even the more technical ones do not necessarily require a full education in the subject: one must be able to understand the technical points, but as one is not trying to make a technical contribution, it is more like being able to read a foreign language without being able to write it, than it is a full back-and-forth translation.