STO was not popular than ICO and it is very rare nowadays to find STO.
Security token offering is same as ICO, but their token type is different from ICO, a security token.
After all, IEO is the most popular way to funding the project nowadays.
Mostly STO was used ERC721
in additional if STO was getting failed caused by the majority of people are not interesting to get the security token. Even if icos can be considered as security tokens and these are still able to provide the utility usecase.
IEO was also the same like ICO but it was getting launched in IEO platform.