The other perspective to add is historic: in the early days of the Macintosh Apple helped make Adobe Type 1 Fonts a printing standard and a deep OS level font standard and then Apple had a long relationship of depending on Adobe for binaries for Adobe Type Manager for core OS experiences. Adobe Type Manager had a long and interesting history, including and in particularly its huge share of OS-level, kernel breaking vulnerabilities. In OS X Apple paid Adobe to finally get the source code for ATM so they could do their own vulnerability reviews and own that code locally. Presumably at great cost.

(Aside: Microsoft got caught in the same proprietary "standards" problem having to ship Adobe Type Manager with Windows for just about the entire history of Windows and then eventually paying Adobe enough to get to the source code and fork their own version of it that they could entirely own for vulnerability scanning while maintaining backwards compatibility. Also presumably at great cost.)

It certainly seems easy to understand why Apple might have been hesitant to depend on yet another proprietary binary drop from Adobe as core OS service in iOS had they embedded Flash into iOS Safari given that expensive history already with Adobe Type Manager.

(Adobe may have indirectly killed Flash just by that history/reputation of being a bad, expensive vendor with tons of vulnerabilities and charging Apple and Microsoft an arm and a leg to fix those vulnerabilities.)