There are two parts to this problem. There is undoubtably an issue of corporate culture that affects compliance with SOPS but there is, I believe, a more important issue: the quality of the company SOPS and regulatory instructions above it.
Where an operation has poor SOPS there is a high likelyhood that people will deviate from them. This results in the normalisation spoken about above.
Poor SOPS can come from a number of factors. But examples I can think of include inappropriate procedures (eg from a different aircraft type that don't apply), procedures resulting from incorrect risk assessment (eg a procedure designed to prevent a risk causes a different, greater one) and excessive procedures.
The last, in my humble opinion, is the biggest issue. We live in a world with a surfeit of lawyers and the answer to most incidents is 'write a rule' to prevent it happening again. This results in a complex interplay of legislation, manuals, memos etc that on paper should make us safer but in most cases rules overlap and contradict. It doesn't take much reading of Pprune to see how wide a variety of understandings there are of what rule applies when, before we even get to what is a good idea!
If we have an operation with a succinct set of rule of what shall and shall not be done coupled with guidance of what should be done and a clear understanding of which is which, it all works. People should and generally will follow the SOPS. But if we have a mixiblob of conflicting, impractical, and incorrect instructions coupled with so many pages of minutiae that no one can remember every rule people won't follow it.