The virus is getting strong because it is mutating and adapting to the environment, its in the biology of the virus to mutate and get better . The newly emerging variants of the coronavirus are due to these mutation only. Plus the virus does not show its symptoms in its incubation time pwhich makes it more contagious. Sars-cov 2 uses a spike protein to attach to the receptor site ACE2 of the host cells which it is attacking. The spike protein is said to be more stable then previous strain that is one of the reasons of its fast transmission.