From what I understand, it is currently necessary for EU countries to join the Schengen Area (src: https://ec.europa.eu/home-affairs/what-we-do/policies/borders-and-visas/schengen_en). I know that some countries (e.g. Bulgaria and Romania) are not yet in the Schengen Area despite being in the EU (in my above example, since 2007). I've been reading up on why this might be the case, and have come to seemingly conflicting information. In some things that I've read (e.g. here: Why are Romania's and Bulgaria's reasons for postponing entering the Schengen area considered non-technical?), the reasons for this postponement seems to come from other countries already in the SA, while other sources posit internal reasons (e.g. "Bulgaria does not want to enter because...).
My question is this: what are the mechanisms the EU has in place to enforce the entrance of these countries into the Schengen Area/what are the time frames for these mechanisms? It does not seem likely to me that these decisions can be put off indefinitely, but perhaps I'm wrong. Any elucidations on this topic would be sincerely appreciated. Cheers.