Lord Brahma is not worshipped like other gods because he’s cursed. There are many stories related to his curse. According to the Shiva Purana, Brahma and Vishnu approached a huge Shiva Linga and set out to find its beginning and end.
Vishnu took the form of a boar and started digging the earth while Brahma took the form of his swan and began flying upwards. But both of them were not able to find the source. But Brahma was not willing to accept defeat and decided to play trick. As he was going up, he saw a ketaki flower and asked the flower to tell lies that he had discovered the beginning.
When he came back and told the story, Lord Shiva discover the truth and cursed him that nobody in the world will worship him.