Many reasons are possible. Depositoins are very expensive. Much information can be obtained from interviews (without a court reporter) followed by a declaration that the witness signs (to help insure the witness does not change their story before or at trial.)
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
It is difficult to answer your question without understanding the context. Generally, many factors can affect the decision who needs to be deposed, including whether that 'witness' has personal knowledge of any relevant facts, the necessary materials have previously been obtained in writing, the litigation budget, etc. Be sure to consult your own attorney to protect your legal rights.
The biggest factor is almost always costs. Depositions can run into the thousands, and that doesn't include attorney fees. Other factors include time and, if the witness is definitely friendly to a party (i.e. the testimony is favorable and predictable), a deposition is probably unnecessary.
The information/answer is not, nor is it intended to be, legal advice. Consult an attorney regarding your individual situation. This attorney is only licensed to practice law in California. Your question and this answer do not create an attorney-client relationship. Do not send/post any confidential information.