Explain why lamarckism is currently not a widely accepted theory

Lamarck's theory cannot account for all the observations made about life on Earth. For instance, his theory implies that all organisms would gradually become complex, and simple organisms disappear.

Lamarck’s theory of evolution, also called as theory of inheritance of acquired characters was rejected since he suggested that the acquired character which an organisms gain through its life experiences are transferred to its next generation, which is not possible since acquired characters does not bring any change to an individual’s set of genes.