Goldman Sachs R&D Report, page-5

The reason CSL112 isn't listed as 40/50% like the typical cardiovascular drug is due to the lacklustre Phase 2 results and the Pfizer Phase 3 results of a similar drug:

"The results of CSL112’s Ph2 trial (AEGIS-I) were published in Dec-16. While they showed that CSL112 infusions were: 1) well tolerated; 2) potentially not associated with adverse side effects on liver or kidney function (though not convincingly so); and 3) increased CEC, we believe the data on clinical efficacy was underwhelming. Not only did the trial not show superiority in reducing cardiovascular death or non-fatal heart attacks, it actually showed a small (statistically significant) increase in cardiovascular deaths in the 6g dose (4/421 vs 0/418 deaths for placebo)."

The report also mentions Pfizer trialed a slightly different HDL targeting drug (different mechanism) in Phase 3 and the trial was "discontinued in Ph3 due to safety concerns raised by the independent data safety monitoring board."

I'd say these are the two main reasons for the conservative 10% chance of success. And honestly this is the nature of the business. Trials fail all the time, the important thing is gain relevant data and reduce the resources wasted.