The way I see it, the people that do get into IB are smart enough to excel in whichever degree they choose. Com/LLB just happens to be the degree most of those candidates choose to do (probably because of its high ATAR requirement since most IB prospects do pretty well in high school as well).
Other benefits are:
1. Decision-makers in recruitment see Com/LLB as a more prestigious degree than other combinations
2. It's 5 years, giving students plenty of time to pad their resume as well as have multiple attempts at internships
In my opinion, the best degree for someone wishing to pursue a career in IB/MBB is USYD's Com/LLB (besides UNSW's Co-op Finance program which gives you 1.5 years of IB experience), since they have industry placement programs as well as their UCLA/Washington D.C. relationship which gives the top candidates a combined ~6 months of experience in their first two years of university.