Lord Brahma and his Temples

According to a story in the Shiva Purana (dedicated to Lord Shiva), at the beginning of time in Cosmos, Vishnu and Brahma approached a huge Shiva linga and set out to find its beginning and end. Vishnu was appointed to seek the end and Brahma the beginning. Taking the form of a boar, Vishnu began digging downwards into the earth, while Brahma took the form of a swan and began flying upwards. However, neither could find His appointed destination. Vishnu, satisfied, came up to Shiva and bowed down to him as a swarupa of Brahman. Brahma did not give up so easily. As He was going up, he saw a ketaki flower, dear to Shiva. His ego forced him to ask the flower to bear false witness about Brahma's discovery of Shiva's beginning. When Brahma told his tale, Shiva, the all-knowing, was angered by the former's ego. Shiva thus cursed him that no being in the three worlds will worship him.

Another reason is...

Legend has it that Lord Brahma is said to be very liberal with granting boons and is easily pleased when prayed to without any thought as to who is praying and asking him for a boon. HE is rather callous in granting boons to the demons unguardedly.

All the deadly demons, from Hiranyakashipu to Ravana, received their boons from Brahma, which made them singularly notorious in destroying the noble virtues of the world. Then it became necessary for Lord Vishnu to appear in his various reincarnations to kill these demons.

That is why the cult of Brahma's worship declined as HE became unpopular amongst religious devotees who began to deem Brahma as the sole God of worship for the demons.

There are very few brahma temples in india..like pushkar temple in rajasthan.... one in kerala... Brahmakapalam and the  brahmakarmali goa.. worshipped from 11th century at carambolim in goa (See below Picture).