MySQL was a serious threat to Oracle: at the time when the Oracle database represented about 80% of all revenues (and the crew needed it to maintain it, even more than the profits).
MySQL creates important adjustments: large Fortune 50 companies with millions of pages that move Oracle databases (especially read only) to MySQL, because the administrative configuration was that great army. I know I helped some of that.
Many in the MySQL community wanted to add features that use a free version of Oracle. MySQL was off at that point. The work groups were carpeted and Larry was afraid.
So Oracle bought MySQL to have control over the market, distribute the community and save the flag of the alien mass.