There are a couple things I would note. Most Classical music is pre-recording technology, so we don't have any recordings of how it would have been played at the time, which would likely be the way the composer would have intended it. Because of this, we have to rely on the notation alone. The older the piece of music, the less standardized the notation practices of the time were. There were also some different standards for different regions, as travel wasn't as common and long distance communication was strictly letters.
You also have to consider that different editors can have an impact on what is being played. Sometimes they add or remove things from certain pieces, presumably in an attempt to live up to the composer's intentions. They are essentially imposing their interpretation of the music onto the score, so you're basically looking at a score that could be "wrong" sometimes. People who are familiar with the different publications of the same pieces will point out these differences and allow a more open debate about the "correct" interpretation.
The older the music, the less likely there will be agreement on how it should be interpreted, especially if it was "lost" for a period of time. Bach's music was famously rediscovered by Mendelssohn. It had been around 100 years since Bach's death before his music was rediscovered, so it was well removed from the traditions of baroque music, leaving a lot of room for interpretation. There are lots of people who dive into music history/musicology and attempt to get a better understanding of different periods, so we tend to see several different sorts of knowledge pools. Some people have a long teaching lineage, so they were taught/are teaching based on a tradition, potential as far back as the time period of the music they're playing. You could argue that those people would have the best insights/interpretations, however, you do have to consider the Telephone Game effect taking place here. The historians and musiciologists would derive their interpretations from research, potentially avoiding the Telephone Game phenomenon, but they're also forced to figure it all out from the writings they can find.
TL;DR: it's old music, we don't always have great references, and sometimes people have imposed their interpretations onto the music we read.