There has been an aspartic acid to glycine mutation on the surface protein of the virus. It’s been shown to be more infective but less lethal, so that’s one reason the virus seems “weaker”. Also evolutionarily speaking, a virus that can spread to more hosts (therefore it has to be not lethal to kill the host before spreading further) will be considered a more “successful” type and is what viruses “aspire” to evolve into.