The energy density of lithium batteries is much higher than that of lead acid batteries. That means for the same weight, lithium batteries can pack a lot more energy than lead acid batteries.
Therefore, mobility scooters can be built lighter so that they can become more portable. The lightest portable scooters that use lead acid batteries typically weigh about 45 - 50 kg, while those using lithium batteries weigh half as much.
Lithium batteries are more expensive than lead acid batteries. So if you buy a lithium battery-powered scooter, not only will it cost more, but you will also need to shell out more money for replacement batteries every 1 - 2 years.
90% of lead acid batteries used in mobility scooters come in 3 sizes: 12AH, 20AH and U1. They are all 12 volts, and are connected in series to power a 24V motor. Not only are they used in mobility scooters, they are also widely used for electric bicycles, electric scooters and back-up power systems.
This kind of standardization is like the AA, AAA or C batteries that we are all familiar with in our household devices such as torch lights and remote controls.
This means that you are not bound to go back to your original supplier to buy the batteries. This gives you the freedom to choose your own batteries (some brands, such as LONG batteries, are more reliable than others). It also means that if you are bringing your scooter to some far-flung town, there's a pretty good chance of you can still find compatible batteries locally instead of shipping it from a far-away country or city.
Lithium batteries, on the other hand, are custom-made to fit the particular model of scooter you have bought, and you have to go back to the original factory or shop to buy the batteries.
Lithium batteries have been known to catch fire due to thermal runaway, which does cause a risk to life and property. Lead acid batteries have no such problem; they do bloat due to age or overcharging and sometimes leak, but they don't typically catch fire.
So if you are buying a lithium battery powered mobility scooter, it is best to choose reputable brands such as Solax.