In a nutshell, it has a unique structure, a wider mandate, and operates a bit differently than your typical intelligence service - especially domestically.
"One of the largest and most secretive intelligence organizations in the world, it maintains powerful branches at the provincial, city, municipality and township levels throughout China."
>operates a bit differently than your typical intelligence service - especially domestically
Fair enough.
But as wild speculation with no real insight, I'd guess that such a large mass organizational structure probably also precludes the same rigid centralization and advanced capabilities of other intelligence agencies i.e. it probably operates more as a snitching hotline to gather less specific intel on a wider range of people with less ability to perform any actionable espionage type operations, instead of a horde of spies illegally infiltrating every western government, university, and business.
"One of the largest and most secretive intelligence organizations in the world, it maintains powerful branches at the provincial, city, municipality and township levels throughout China."