Because Lithium ion is not that available, in many places they are really tough to find. They need to be imported from some other place so they are not used that much.
Price is also another important thing to consider here, price of lihtium ion cells are significantly higher than Lead acid/sealed lead acid cells, yes lithium ion will give you more cycle and will be less bulky but sometimes people care more about the money because in many instances, even in my home all the inverters has tubular lead acid batteries because they are rarely moved. So mass is not the factor.
Another reason is lead acid batteries are here for a while, within this time huge amount of experiments have been conducted on them and we know about them very well and charging circuit for lithium ion is much simpler than that of lithium ion so many manufacturers tend to use lead acid batteries.
And the most important thing maybe is the fact that lead acid batteries works just fine, the 100Ahr batteries that we were using,they lasted for about 5 years each which is an awesome value and I’m sure many of the manufacturer and user will share the same view so I think these are the reasons why all of the inverters are not using lithium ions.