As I've said in other questions, people's individual reasons for joining IS will be different. But there are huge and basic differences in what these two organisations are.
Saudi Arabia doesn't fight the West
In the middle east a lot of Muslims hate the West, especially the US. The way they see it, there's a conflict going on between Islam and everyone else. Not necessarily a violent conflict, but a conflict nonetheless. I don't think this is a majority view (most people probably don't care) but it is a significant minority that feels this way.
So some think there's a conflict, and some of those think there's a violent conflict. Some of those will be willing to do something about it, and Saudi Arabia is hardly the poster child for Jihad.
The sad fact is that these people have an ideal to fight for, something they're willing to die for. That willpower makes them dangerous, so they turn out to be an effective fighting force. They've been able to make incredible gains with a small number of people. This has a theological advantage too- I'm sure their recruiters use it as a sign from Allah. It bolsters their theological claims and makes people want to be a part of it because people want to win.
Many of Saudi Arabia's policies aren't deemed to be extreme enough
Saudi Arabia's policies are pretty oppressive, but they've been getting slowly more liberal over time. To those in the West it's a subtle difference, but I imagine it's quite stark to the inhabitants of the region.
Islamic State is claimed to be the Caliphate
This is probably the most important reason. Saudi Arabia has never claimed to be the Caliphate, whereas Islamic State has declared that they are. That means that they have claimed that their leader is the leader all Muslims, everywhere. That's a grand claim that essentially declares their intention to impose Sharia over the entire Muslim world.
They actively recruit in other countries
Islamic State has been extremely skilled at using modern technology to recruit people. They use social media, their recruiters contact people, and they build up a movement in other countries. They spread their ideology, deliberately, and make it practical for people to join them. If you merely wanted to live under Sharia (as many do), you probably wouldn't know how to do that. It would just be a distant dream. But if you have someone contacting you, telling you that you can make a difference, giving you directions, then that's very different.