SLV was more of a technology demonstrator, what were we going to do by launching 40kg to space the payload was not meaningful enough.
Also it had just 4 launches
2 were successes and 2 were failures.
SSLV is being made with a different goal in mind, it is more of a launch on demand rocket to provide frequent launches, the launch vehicle's technology is offered to private sector companies so that they can compete in the global launch on demand market.
It's a 4 stage solid propellent based launch vehicle that can be manufactured in less than 2 week and can be launched by just 6 people.
Capabilities 500kg to LEO and 330 kg to SSO.