Maybe they use dilithium on ships that didn't use forced quantum singularity as power sources and they could be using it for trading. Plus dilithium may have other uses outside of regulating M/AM reactions.
I would surmise that when we saw the Romulan Warbirds in TNG, it may have been the first instance of Forced Quantum Singularity as a power source appearing.
Obviously, all Warbirds would be using them... but perhaps not every single ship in the Romulan fleet (for example, we don't know if their scout ships use the same technology... and who knows how many other classes of ships they had - so, apart from the D'Deridex class warbirds, maybe no other ship was using Quantum Singularity as a power source).
The Narada for example wasn't mentioned that it has forced quantum singularity for a power source - but the 2009 movie wasn't that big on giving us technological details (sans the Transwarp beaming which Scotty managed to get working in late 24th century... but of course, that also doesn't apparently exist in the 32nd century - or at least we hadn't seen it in use yet).
Each Warbird needs to come with an engineering manual to allow the chief engineer to help repair it - and I doubt the Romulans lost all their D'Deridex class ships or simply phased them out of service given how massive (and as such useful) they are.
We also don't know if the Warbirds from Star Trek: Nemesis movie used FQS or M/AM.
Though, the technology could have been suppressed by the Tal'Shiar (which could have kept it for itself) when Shinzon took power, and after the events of Star Trek Picard, it may have faded into history - plus with the reunification with Vulcan in the next few centuries, the Romulans may have decided to 'let go' of their empirical aspects and technology (leave it behind).
The Romulans had several major events which upset the balance of power against them:
1. Shinzon coup and the Scimitar (which was a ridiculously overpowered ship - not impossible to make given what Trek technology is capable of mind you, just that because of how powerful it was, it would have been able to go against small fleets of ships and win... which means that Shinzon could have crippled the Romulan fleet more or less - and the Scimitar being built in secret on Remus, likely used Dilithium and M/AM).
2. Supernova which destroyed Romulus star. This would also push the Romulans to the brink... especially if most of their production facilities were that star system (although, this makes little sense for a space-faring species that calls itself an EMPIRE - which implies they would need to have production facilities throughout their empire).
As for why Starfleet never used it... writer choice... and beats me.
There are multiple power sources they could have pursued over M/AM and dilithium right after Voyager got back, but apparently, this never happened (probably for the purpose of 'drama' to make the Burn work).
Even the fact that Starfleet knew during TNG already that FQS technology existed (they knew Romulans had it) they should have been able to make it themselves in the next few decades (maybe 50 to 100 years maximum given their level of technology and overall knowledge).
This is likely the case of writers not paying enough attention to Trek history (even though its not that difficult to look up), and then taking into account the fact that 930 years (after mid 23rd century) is a ridiculously long amount of time... time enough that Starfleet would have been gone beyond dilithium and M/AM by late 25th century realistically.
I'm sure though the writers will undoubtedly say that FQS also needed dilithium to regulate energy... which doesn't make sense because dilitihum was specifically mentioned that its needed to regulate M/AM reactions... FQS wouldn't need that.
@GeneralMartok12
I'm also from Croatia. :-)
And good question, but honestly, Trek writers, consistency, and viable explanations seldom go hand in hand.
