It's sort of hard to know what happened in more detail without really good-quality reporting from in the courtroom, that might inform as to why the jury found the way they did. We know that the judge issued a majority instruction, a not very uncommon process where, if the jury deliberates for too long, the judge tells them that a ruling can now be accepted with one or two dissenters. I don't think the jury says what the vote was, just that it was enough, so it could have been unanimous, we don't know. In the US journalists often try to track down jury members and interview them to get those kinds of details but jurors don't always want the media circus around them and I don't know if that's common practice in the UK.

The dropped charges were apparently alternative counts covering the same crime as the other charges and were probably dropped for that reason, although it's interesting that no reporters seem to have really given a reason why. I don't know enough about the UK legal opinion to give much of a guess as to why except that I see the crown prosecution manual does discourage using alternative counts (of a lower crime) in rape cases. Some context in, of all things, Yahoo News, suggests the judge may have been concerned that the alternative counts were making things more complicated for the jury. In the US, some states prohibit charging the same crime as multiple alternative counts for this reason, or have a special procedure for the jury to make it easier, it's sort of a known issue that it's hard for a jury to come to a verdict when they're having to choose guilty/innocent for multiple counts of different exact allegations over one event.

The outcome is dissatisfying, but it's sort of the outcome the criminal justice system is designed to produce. All the jury found is that there was not evidence beyond a reasonable doubt that Spacey was guilty. There could still be evidence making it most likely that Spacey was guilty, but criminal prosecutions require the higher standard because of the severe impacts of criminal charges. The crown prosecution service put out a statement that they accept the ruling and were just doing their job, which in the US would look more like a prosecutor trying to save face over pursuing what wasn't the strongest case, but I have no idea about the UK, it might be totally routine to make a statement like that.

The Spacey thing is interesting if you like to follow the legal details. The US cases against him (criminal and civil) were both dropped after the accuser stopped cooperating with the prosecution, at one point pleading the fifth when testifying. That tends to be taken as a sign that the accuser was lying about at least something substantial, thus the prosecutor dropping the charges, but that wasn't determined by the court or anything. It's possible, although maybe not so likely, that the accuser was acting in good faith and lost interest in pursuing the case for some other reason. That's probably more likely when a celebrity is involved, these celebrity prosecutions are all the more complex when it comes to people's motivations.

The whole culture around sexual harassment/assault allegations has changed a lot over recent years (since "MeToo" if you will), for the better in many ways, but I think there's still a lot of unsettled issues. It seems like in celebrity cases it's a lot more likely that "hangers on" will show up with claims that are maybe not untrue but at least stretch the truth. At the same time we obviously have to take accusers very seriously or we risk ignoring criminality because the accused is a popular celebrity. The justice system produces a lot of these unclear outcomes where maybe there were multiple accusers and things look really sketchy for the accused, but it's not clear enough for a prosecution. That situation has always ended up going to the court of public opinion for a final verdict, but in the case of a big celebrity like Spacey that's sort of a huge deal. Will studios keep working with Spacey? I suspect the answer is yes, because he's a huge celebrity, not because of anything about the merits of the case against him. In corporate environments the company will sometimes hire a private investigator to make a decision about the accusation and fire based on that (this is in the US where there are no employment protections), which kind of has its own sketchiness, but the media industry isn't known for caring that much.