The pathogenesis of the SpA and the reason for the tropism for the sacroiliac joints is still obscure. It has long been suggested that a genetic background of an initially bacterial triggered infection seems to be most likely. Whilst the synovium is the primary site of joint disease in the rheumatoid arthritis (RA) the primary site in spondyloarthritis ‘SpA’ (including Ankylosing Spondylitis; AS) is less well defined. It has been proposed that enthesitis-associated changes in SpA are primary and that all other joint manifestations are secondary. Nevertheless, some of the sites of disease localisation have not been adequately explained in terms of enthesitis. A reasonable hypothesis is that inflammatory responses characteristic of SpA are triggered at these seemingly diverse sites, in genetically susceptible individuals, by a combination of anatomical factors which lead to higher levels of tissue microtrauma, and the deposition of microbes.
These old German articles were trying to reach an answer or hypothesis:
Curr Opin Rheumatol. 1996;8(4):275-87. The sacroiliac joint in the spondyloarthropathies. Braun J, Sieper J. https://www.ncbi.nlm.nih.gov/pubmed/8864578 Rofo. 1997;166(2):95-100.
Sacroiliitis: the key symptom of spondylathropathies. 1. The clinical aspects. Bollow M, Braun J, Hamm B. https://www.ncbi.nlm.nih.gov/pubmed/9116265
However, years later in the UK, the following interesting review article tried to throw light on the insights of your question from an anatomical perspective:
The anatomical basis for disease localisation in seronegative spondyloarthropathy at entheses and related sites. Benjamin M, McGonagle D. J Anat. 2001;199(Pt 5):503-26. Review. https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1469-7580.2001.19950503.x
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