Lead has an incredible lifespan; it is not unusual for lead flashing to last over 100 years. Unlike all man-made alternatives which you would be lucky to get 10 years lifespan before they fail.
Lead is very malleable making it easy to bend into many difficult shapes and sizes to suit every application. Unlike hard metals which require special machinery to fold.
As lead can be bossed or cut and welded, it can overcome pretty much any detail whilst not compromising the waterproofing and effectiveness of the lead. This cannot be said about lead alternatives, even copper, which is also a premium product, struggles when it comes to technical details.
Whilst the initial outlay of installing lead flashing is generally more expense than other alternatives, due to the life span of the lead in the long run it actually becomes a cheaper more cost effective option.