Did you ever notice the best people in the industry dont just do every job someone throws at them. They can be picky because there stuff works, if you very from what they figured out it may work or may not but why would they want to take that chance?
Does SLP make alot of stuff for Carls? Sure. Alot of it is different designed stuff you can only buy from carls not from SLP or anywhere else, its there own design!
Why would you want to use Bobs head, Steves Cyliders, Harry's pipe, Joes reeds??? Did you figure out something none of these other companies figured out?

or are you just guessing?

No reputable company wants to take a chance of getting bashed over 1 part they worked on and you used 3 other peoples stuff and now your not happy and then you go bash them more than you already are because they refused to do your work because they dont want to take that chance.
Carls obviously made the right choice here.
If you want a combo that works get there combo all from them, I know it works and so do they, if for some stray reason something is off they can fix it really easy!
Carls is probably the best Polaris complete Snowmobile mod shop for Polaris mountain sleds. If you use there whole package you will be happy. Jack test every package until he is content with it, until then no consumer will get to try it.
Trigstad is probably arguably the best skidoo motor guy. If you dont use his complete combo he doesnt do a thing for you period. Why? because he knows it works and he knows you will be happy.
Its quality control! More companies should be this way!