Google decides a page is worth indexing when it judges the page offers unique value worth storing and serving, which in practice means three things together: sufficient quality, distinct content rather than a duplicate, and enough trust to be worth showing for some real query. That combination, unique value plus quality plus trust, is the criterion. It is what the system is looking for when it decides whether to keep a page in its index, observed across how borderline pages get treated, and it is not satisfied by any single technical switch.
Unique value means the page brings something a searcher could not already get from a page the index holds. A near-duplicate of existing content fails here even if it is well made, because storing it adds nothing. Quality means the page is genuinely useful and competently made rather than thin or auto-generated filler. Trust means the page and its source are credible enough that serving it would not degrade results. A page has to clear all three at once; strength in one does not excuse a gap in another.
It is worth being clear about what this criterion is and is not. It is the standard the page must meet, not the set of moves you make to meet it. Believing any published page gets indexed misreads the system, which routinely declines to store pages that offer nothing distinct. Pointing to one technical trigger, a sitemap entry, a submission, an internal link, also misses it, because those help discovery but do not make a worthless page worth keeping. The criterion is about the value the page carries, judged on its merits.
For your next page, test it against the criterion before you expect it to be indexed. Ask what unique value it offers that no existing page already provides, whether its quality is genuinely sufficient, and whether the source carries enough trust to be served. If it cannot answer all three honestly, indexing is unlikely, and the fix is to give the page real, distinct value rather than to look for a switch to flip.