One of the reason might have been vanity but if you look at it with the view point of a CEO from a European firm, reusability was a risky gamble:
1) In order to keep your manufacturing quality, one needs to produce a new rocket every two or three months. Beyond that, the people manufacturing it losses knowledge and unless quality assurance is increased a lot, you will losses quality.
2) EU was doing around 12 launches a year, and that was by being very cheap. You could plan on increasing the market share but that starts being difficult as a lot of what remains cannot use a European launcher for political reasons.
3) For the sake of the argument, let say cheaper price double the market share, that leaves 24 launches a year. Based on a manufacturing of 6 launchers a year, this leaves 4 launches per rocket. And that is with very optimistic numbers (doubling of marketshare, no issue with reusability).
4) If you look at the numbers from SpaceX, 4 launches per rocket is where the reusability starts making sense. But ideally, you want more. 5) The Airbus people have shareholders to account to, not a CEO who does not care risking going banqueroute while pursuing a dream to go on Mars.
So yeah, doable but much more difficult than in a market with better access to investment and to huge cash cows named NASA and DoD