Additionally, Monero has a culture of frequent hard forks that simply does not exist in Bitcoin.
This is the reason, why they did it in a hard-fork way. Because their community is used to that approach. The same rule applies for a lot of altcoins: they could use soft-forks, but they don't have to, and don't want to, so that's why it is what it is.
This has been a monero marketing point for a long time. To some extent I think it's kind of pointless navel gazing.
In general, you can follow the whole discussion between Peter Todd and Gregory Maxwell, to get the full context.