You ask why the
url fields don't show up in the formatted bibliographic entries when you use the
acm bibliography style.
The direct answer is, "because the
acm bib style is programmed to recognize and process just the following field types (listed alphabetically): address, author, booktitle, chapter, edition, editor, howpublished, institution, journal, key, month, note, number, organization, pages, publisher, school, series, title, type, volume, and year". Any and all fields names not on this list get ignored.
You might be tempted to ask, "why wasn't the
acm bib style programmed to recognize some other fields, such as
url? The answer is surprisingly simple: because the
acm style has been around virtually unchanged since 1988, when most people (including, evidently, Oren Patashnik, the creator of BiBTeX and quite a few bib styles) had never heard of dois and URLs.