In Europe, Android dominates the smartphone OS market with ~75% market share and most come pre-bundled with Google software. With names like "messages", "photos", or "calendar" it's not really clear that these are Google products or that they can be changed. You basically end up with most of that market share using Google software without even understanding that they have a choice.
I believe the logic behind the fine was very similar to the USA vs Microsoft anti trust suit (https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor...).
To put it in perspective though, the fine Google received was less than 2 weeks worth of revenue and will probably be lowered if they appeal. It's not really as big a deal as the media made it out to be.
I'm not saying I agree with the fine, but I do understand how, in a certain light, the EU see Google's practice as a bit monopolistic.