Lots of speculation here, but:
Probably one reason it’s taken so long to implement quote posts isn’t because it’s all that hard to pull a preview and display it when embedding a link to another post, it has to do with notifications.
And likely also things like filters.
For instance, if I block/filter out the original post, are all quote posts of that post also filtered?
I mean, I’d certainly hope so.
And then there are the people who don’t want quote posts at all. Will there be a profile setting to disable the ability to quote post anything? Will it only be enabled for Public posts? Will it work across servers (ie., if you reply to a post on some other server, will your quote post settings work there, too? Will this be honored by other fediverse software?)
Lots of potential factors to get right here, depending on how much work they put into the implementation.