Look at something like Active Directory that is in theory ldap.
Do you really want to mix AD and open LDAP? You can, and it's a massive pain in the ass. Now you have apple iWatch apps talking to android. Where does apple maintain the translation layer? Does google/samsung take a "fuck off" stance and lock them out? What does the user experience look like when the "bug" in the app is on android and your CALLING apple for support...
I think that last bit is the limit that folks dont really think about. You can call apple, show up at store and they are going to try to fix your issue... when your random android device breaks who do you call?
"the technical limitations" are "we cant control android" and "does not fit our customer support philosophy"