Hello,
The main reason is that the two types of boards have different functions. The line board works between the cross-connect board and the demultiplexer board. Provides WDM-side interfaces. The tributary board works between the cross-connect board and client equipment and provides client-side interfaces.
The following uses ND2 and TOA as an example:

For details, see the working principle and signal flow of the board in the product documentation.
Thanks