What @Priller said 
There is no intentional discrimination again certain types or manufactures. It’s just a culmination of the things mentioned by @Priller. Also keep in mind that all our data is researched primarily by volunteers, so let’s cut them some slack, shall we?
I might re-iterate one of @Priller’s points: AirlineSim’s current performance formulas are - how shall I put it? - “not ideal”. Both payload and take-off/landing performance formulas require dirty hacks in the data to get anything close to real-world performance. Without better formulas, we’ll never get better results and there’s also the question of how much sense it makes to invest in fiddling with the old data if new formulas are on the horizon. Because Improved Aircraft Performance formulas are on the agenda. And if this is important to you, cast your votes there