An MCA is a postgraduate degree, so employers expect more than they expect from a BCA — and that expectation is the whole of the salary difference. The syllabus itself is deeper, but the market does not pay for syllabus. It pays for what you can do on the first day.
The spread sits almost entirely on one question: did the two years add practice, or only papers? An MCA who spent those years building, contributing to something open-source, or interning seriously is a straightforwardly strong hire. An MCA who spent them passing exams is competing against BCA graduates for the same ₹3 lakh roles, having paid two years for the privilege.
There is also a quieter split by route. MCA through CUET PG into a well-known department puts you in front of recruiters who visit; MCA from a college nobody visits means you are applying off-campus, which is slower but by no means closed.