Brahma is not worshipped because of a curse by the great sage Bhrigu. The sage was organising the great yagna and invited all the gods. When he went to invite Brahma, he was so busy with the music played by Saraswati that he could hardly hear the words of the sage.
This enraged the sage and he cursed Brahma that no person on earth will worship him.
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.