"most mature" != "best"
Lead-acid is the oldest technology, is still in current use and so also has had the longest
period of continuous production (>200 years). A larger number of units have been constructed
than for any other technology in comparable roles. Performance, longevity, reliability have all
very much plateaued. Manufacturing capacity already exists in line with demand, with sourcing
of materials based upon well established pre-existing industries. Applications have included
everything from low powered consumer electronics to large scale grid energy storage systems.
Actually zinc-chloride, acetic, sodium-salt, and ferrous-phophate batteries predate alkalines and lead-acid by decades, centuries, several decades, and years respectively.
Not counting capacitor-esque stuff since they wouldn't be able to run actual machinery or lights for any length.