Well, there's a lot. We don't know much about early Rome because there's not a great historiographical record left behind. We rely heavily on works written long after these events took place. People were writing, including those in Rome and those outside of it, but not all these writings survived into the modern day. Some of them may have been surpassed by later written works, some of them may have been a bit boring and just not enjoyed. We know that some of these works still existed when others were writing theirs, so they are maintained in that way (i.e. in fragments) But we run into a whole different set of complications when evaluating fragments.

Still, people in Rome during its heyday, and historians ever since, have done a good job at piecing together the little bits of information we do get to try and at least paint a picture of what was going on, even if there are still a lot of gaps, a lot of questions, and -- to a certain extent -- a lot of guesses.

We know tons about ancient Rome. There are literally anthologies written about it. If you want to learn more about ancient Rome, I would suggest starting with The Storm Before the Storm by Mike Duncan in audiobook (he both wrote and narrates the audiobook). He had a long running podcast about ancient Rome as well. With regards to the decay of written accounts, I always remember what one of my professors for a Historical Archives class said in college: imagine that for each century that passes, 9 out of 10 historical documents and artifacts that remained were destroyed, melted down, or decayed. Some from that century might write new copies or accounts, or replicate the paintings or sculptures, but these would no longer be the originals. After centuries of loss, very little of the originals would be left, just a tiny remnant. The rest would only live on in archival copies or a reference in another work. This is why places like Pompeii are so important, they are like time capsules preserving the history (until we dug it all up). The last few centuries were particularly bad about chopping up historical artifacts and dragging them across the world to museums, and wars destroying historical landmarks.

Following the fall of (Western) Rome, the economy cratered and the money that went into maintaining the academies where copies of written works were made disappeared. Books do not last forever, and in those days they had to be copied by hand, an expensive and time-consuming enterprise. The people who were literate in those days were Christian scholars who didn't have much interest in preserving historical and scientific tracts.