It is not completely true that Lord Brahma is not worshipped. There are some temples in the world where he is worshipped. He is also worshipped at the start of some Hindu rituals in the form of a Kalasha. But yes, compared to the other gods, he is less worshipped.
Why is Lord Brahma Not Worshipped?
Most of the Hindus think that Lord Brahma is not worshipped because he married his daughter, Saraswati, but it is partially true. There are at least four stories behind why this one of the main gods of Hindus is not worshipped.
1. Goddess Saraswati Cursed Lord Brahma:
Once, Lord Brahma had arranged an important yajna, which needed his wife to accompany him, but could not start the ritual because Goddess Saraswati had not arrived yet. Therefore, he consulted with the gods, who advised him to create another wife for him. Therefore, he created Goddess Gayatri and completed the ceremony with her.
When Goddess Saraswati arrived at the scene, she could not control her anger after seeing the second wife of her husband and cursed Lord Brahma that he would not be worshipped by humans.
2. Lord Brahma Lied (Linga Purana 1.17.6):
At the end of the Kalpa (a period of 4.32 billion years), there was only a single vast sheet of water. Lord Brahma saw Lord Vishnu having a yogic sleep on the bed of Shesha (a serpent deity of Hindus). He woke him up with the blow of his hand and asked him who he was. Lord Vishnu told him that he was the creator, sustainer, and destroyer of the world. It infuriated him because he believed that he (Lord Brahma) was the creator, sustainer, and destroyer of the world. Then, they got into an argument about who was superior.
The argument turned into a furious fight, and then suddenly, a Jyotirlinga (a huge infinite pillar of light) appeared in front of them. It had thousands of clusters of flames. Also, it had no beginning, middle, or end. It was the source of the universe. They were deluded by it. They forgot their fight and decided to test it. Lord Brahma assumed the form of a swan and went upwards. Lord Vishnu took the form of a wild boar and went downwards.
They both traveled for 1,000 years but could not find the end of the lingam. Therefore, they returned to where they started. They bowed to the lingam and wondered what it was. Then a loud sound “AUM” came out of the column, and the letters “अ” “ऊ,” and “म” (“A,” “U,” and “M”) appeared on the lingam. Above those letters, they saw Lord Shiva along with Goddess Uma. Lord Shiva told them that they both had been born out of him but had forgotten about that.
Lord Vishnu accepted that he could not find the end of the lingam, but Lord Brahma lied. He said, “O Hari, the top of this column has been seen by me. This Ketaki flower is my witness.” The Ketaki flower repeated the falsehood endorsing the words of Brahma in his presence. Lord Vishnu, taking it to be true, made obeisance to Lord Brahma. He worshipped Lord Brahma with all the 16 means of service and homage.
Upon hearing and seeing this, Lord Shiva got upset with Lord Brahma, and he created Bhairava from his eyebrow hair to punish him. Bhairava cut one of the five heads of Lord Brahma, who was guilty of lying. Seeing this, Lord Vishnu felt pity for Lord Brahma and requested Lord Shiva to forgive him. Lord Shiva accepted his request and spared the life of Lord Brahma, but as a punishment, he prohibited his worship. That is why Hindus do not worship Lord Brahma.
3. Lord Brahma Married his Own Daughter:
According to the Matsya Purana, Goddess Saraswati was born from the limb of Lord Brahma; hence, she is his daughter. But she was so beautiful that he lost his control and started looking at her with lust. To hide away from his amorous glances, she moved to his right side, but he grew another head and continued staring at her. Then she moved to the other two sides, but he grew two more heads. As a last resort, she leaped into the sky, but Lord Brahma grew the fifth head looking at her. Therefore, left with no choice, she agreed to marry him and became his wife.
Per another version of the story (Shiva Purana Chapter 2, Section 3), when Lord Brahma grew the fifth head Lord Shiva appeared there and cut it. He told Lord Brahma that it was immoral to lust for his own daughter. Therefore, henceforth, he would not be worshipped.
4. The Curse of Bhrigu (Padma Purana Section 6, Ch 255):
One day, on Mount Mandara, all the great sages had assimilated for a sacrificial session. After the session, they discussed who is the greatest among the trinity, but they could not come to a consensus. Therefore, they requested Sage Bhrigu to meet the trinity and find who possessed the pure sattva guna, would be friendly to Brahmins, and would be a holy place and favor for Brahmins.
After cursing Lord Shiva for not paying attention to him, Sage Bhrigu went to Brahma Loka. When he saw Lord Brahma, he saluted him by joining the palms of his hands, but Lord Brahma neither rose to honor him nor did he speak pleasant words.
Seeing him with a preponderance of rajas, the great sage, the grandsire of all worlds, who was very lustrous, said (these) words to Brahman: “Since you, having a preponderance of rajas, are slighting me, therefore, be dishonored by all people.”
After reading these stories, you would understand that Lord Brahma marrying his daughter is not the sole reason behind Hindus not worshipping him, but it might be one of the reasons. It depends on which story you believe.