Why is Google Scholar popular in some fields but not in others?

Math already has an excellent tool, MathSciNet. And, more importantly, MathSciNet is much more accurate than Google Scholar.

If you are a mathematician, look up yourself in both places and compare!

In computing science, papers are reviewed rather quickly (because they are often tied to conferences, with deadlines, and without journal backlog). They are also usually indexable.

In biblical studies, to give another example, it can easily take over two years from submission to publication, even with a minor revisions review shortly after submission - just because journals have a large backlog. Also, there still are journals without digital edition. Biblical scholars I know rely on mailing lists for tables of contents and find PDFs of articles they want to read on academia.edu. Google Scholar simply does not index everything and is not fast enough.

From what I've heard, in math it's the review cycle that takes long. This is different from a journal backlog, but the result is the same. Arxiv will be more up to date than Google Scholar.

There is no single answer to this question. Every field is unique. There is also an element of randomness: If a handful of the right people create Google Scholar profiles, others will follow and adoption will rapidly spread. However, I think there are two key factors:

1. Willingness to adopt new technology. This depends on the existing alternatives (e.g., MathSciNet) and also how tech-savvy people are. I've noticed that, even at the individual level, there is a strong correlation between having a good, up-to-date website and having a Google Scholar profile.
2. Publishing attitudes. Google Scholar is very inclusive in indexing. It counts citations from arxiv preprints, not just peer-reviewed journals. I even get citations from lecture notes and research statements that people have uploaded to their websites. Different fields have different attitudes about what should "count" as a citation and that will affect how they view Google Scholar. For example, math is much more conservative than computer science and Google Scholar adoption reflects that.

A final note specific to computer science: Just about every computer science researcher knows people (e.g. former students) who now work at Google. And Google also produces a lot of computer science research. Thus computer science has a very positive view of Google, which may not be the case in other fields.