Promoting your own services in general is fine. But when a huge, huge majority of the market uses your service for discovery, your decision to promote your own services over competitors' damages competition in the market — users end up with less choice in practice, even though they could theoretically switch to another service.
Those rules exist to try and prevent this sort of abuse happening.
If they had competed and won this share fairly, they wouldn't get a fine.
Remember that EU, during these years was consistently the largest, or the second largest, depending on year, economy of the world, so this 80% is huge and justifies the size of the fine.