Apple never adopted it, which meant that you had Windows + Android able to use Miracast, but iOS not; AirPlay couldn't be used by the other side of the fence.
DLNA could, in theory, be all things to all people, but like similar "all things" it's such a pain in the arse to get renderers and servers all talking to each other it never really "just works".
Of course, all of that wouldn't necessarily be a problem except that Apple and Google are deeply invested in non-compatibility.
Apple already had it's own standard in place before Miracast was created.
The audio only version of AirPlay (AirTunes) goes back a decade before Miracast, and the version that also supports video predates Miracast by two years.
It would be nice to be able to add an iOS plugin to add system wide support for Miracast instead of just having some apps choose to support it.
DLNA could, in theory, be all things to all people, but like similar "all things" it's such a pain in the arse to get renderers and servers all talking to each other it never really "just works".
Of course, all of that wouldn't necessarily be a problem except that Apple and Google are deeply invested in non-compatibility.