*A is a big alliance.
within the alliance, there are some airlines that work well together (Joint Venture like LH+UA, LH+ANA, or just deeper partnerships)
https://www.lufthansagroup.com/en/co...-ventures.html
TK just doesnt have a deeper partnership with NH or LH (points 1 and 2).
overall, *A purpose is for marketing. you get your free bags, priority boarding, lounge access. *A doesnt mean that one *A carrier can issue tickets for another *A carrier
(3) may be due to technical issues (*A carriers may know its difficult to endorse ticket to TK for various reasons), even though according to *A guidelines they should look at baremetal airline, then *A, then other airliners during IRROPS)
Reference Guide - Airports Information - Irregular Operations Handling
Carrier Order
Rebook in the following carrier order, depending on frequency of flights and the customer’s best interest:
Original receiving carrier
Own airline/Carrier Group
Star Alliance member airline
Non-Star Alliance member airline (OAL)
eg: united and SQ are in *A, but they are known to be frenemies... you can only issue fullfare SQ tickets on UA