This is a subjective question, but common reasons I have heard (and reasons I would have) include:
- Fear of students editing records to manipulate what you said
- Fear of having a mistake you made taken out of context and spread widely
- Concern that it encourages students to not pay as close of attention
- Concern that students will not feel they need to attend class if they have acceess to the lecture outside of class.
- Concern that students in the class will not feel as comfortable asking questions or contributing to discussion, since they are also being recorded
- Institutional concern about a course's lectures being widely distributed online, when in fact these lectures are for (paying) enrolled students--a type of "intellectual property" concern
Edit: Also, you might want to be aware when bringing up problems getting easily distracted in the class, a professor could take that somewhat personally. Many committed teachers go out of their way to try to make their class as interesting and engaging as possible. If a student asked to record lectures because they are getting distracted or zoning out during class, one may not hear: "I am trying to do this to improve my learning;" but rather: "I am trying to compensate for your short-comings as a teacher". Not all instructors will feel this way, but that's just something to keep in mind as you broach that conversation.
Edit2: There's a lot of discussion in comments that my last comment about socially navigating the question: it is very true that many teachers may not care. However, trying to frame the request more explicitly as "this is a thing I am doing to improve my learning" could be beneficial in making your instructor agree.