Generating and maintaining individuel company SOPs costs time and money, and it requires of course dedicated personnel for that as well. I believe at Lufty around 120 employees worked in the relevant department, over all their fleets. In our case that number was much lower of course.
If a company uses the airbus apps, in our case the flysmart suite, there is no way the airline can edit content themselves. It has to be done by airbus and distributed for each MSN individually, which can cost several millions a year as airbus charges extremely high premiums. Not to mention that the process is designed in the typical airbus way: time consuming, complicated and built with a lot of unnecessary but required work. Therefore keeping SOPs that are different from the OEM set is economically challenging.