Lots of reasons - lead hydrate (white lead, or hydrocerrusite in the raw mineral form) was naturally found in several places in the Mediterranean where it was first used in and probably before the 4th century BC. It was easy to make from mined lead, ground up easily to mix with oil to make paint, mixes freely with other pigments and solvents, bonded better to wood than other available finishes, was washable and retained its bright white color without fading, is highly fungal resistant, was only about a tenth as permeable to moisture as other paints, dried quicker and smoother, and resists cracking and peeling. Generally, was the best type of paint for a long-term durable finish up till the 1980's or so when reliable epoxies came out, and is still used for non-residential uses where wear resistance and long life are needed, like some boat and industrial paint uses and pavement markings for example. It was (and still is) also used for oil painting by artists as the only pure white pigment - ceruse. White lead was also commonly used (and still is in parts of the world) as a bearing lubricant, and was used in medicines and cosmetics until about 20 years ago.
Yellow lead (chrome containing) was and is used to provide bright yellow pigmentation in industrial paints, and Red lead (lead oxide) was commonly used to provide orange and red pigments, and until a decade or so ago was the standard anti-fouling paint for ships below the waterline, as it was the only product that effectively retarded barnacle and other marine growths on ship bottoms. That is why almost all ships had red hulls below the plimsoll line - the maximum loading line.
Up till the 1978 banning of lead paint in homes as a result of studies from the late 50's through mid 70's into child health issues resulting from eating paint, white lead based paint was THE paint of choice by professional painters because it FAR outperformed the other alternatives.
Contrary to a lot of the misinformation and alarmist presentation by the press, lead based paint is not a significant health hazard unless it is ingested or breathed in in quantity as a result of sanding - which is why encapsulation in other scratch and peel resistant finishes is allowed as a remedial solution, as an alternative to removal. In fact, in a residential environment, removal of lead paint from the surface (as opposed to removing the entire surface by removing drywalll or wood) has been shown to commonly be, under normal field working conditions, more dangerous than leaving it in place and painting over it.
