You can get more citations from books because they covers more topics than articles. Books can also be published online to make them accessible to everyone.
Most authors when citing from books does not specify what is taken from that book or mention the book as a general source only and therefore the number of citations in books less than for articles.
Journal articles usually get more citations. Accessibility is the problem. However good your chapter is, if readers don’t have access to the book, they won’t find it. Journal articles are usually available and accessible online.
Well, many journals are only accessible for a person in case this person is registered in an institution (usually a University) that pays publishers (e.g. Wiley, Elsevier, etc.) to have access to its articles.
As you know, researchers read journal articles more than books, because reading them does not take much time. In addition, researchers have time constraints. As a result, journal articles are more cited than books.