We don't have any proper literature left on Mahayajna like darshapurnamasa, niruDhapashubandha, jyotishtoma, aptoryaama, vaajapeya, paundrika, rajasuya, ashvamedha etc. There is no one today who can do these mahayajnas. I am interested in knowing the reasons why these Mahayajna banished?
Vedic yajnas are not banished or banned per se. We still have vedic yagas going on in India.
The scriptures for these yagas are usually shrauta sutras which come under vedAnga.
For example, kerala is famous for most ancient vedic ritual alive (agniSToma). Similarly other types of soma yagas are done in maharashtra and Andhra pradesh. In one of my answers I have mentioned about various types of yaagas.
One of the main drawbacks is availability of SOMA plant. According to some it is extinct now. Different people have different candidates for soma and so on. Soma juice is key ingredient of soma yaaga.
The problem however, is not because of banishing but due to the complexity of the rituals and very high prerequisites for the priests. Because of these requirements, its hard to find candidates who can qualify for conducting soma yaga.
Some of the requirements are:
He should be married with spouse alive
He should have completed AT LEAST samhita studies of the veda
He should be doing daily rites including the agnihotra. (Extremely difficult to maintain in today's society)
He should be doing other routine yagas such as paaka yajna, havir yajna, chAturmAsya, monthly iSTi sacrifice and so on.
He should not borrow money in order to conduct the yajna (somayaga may need a million or two) . Apart from the yaga he should have savings which is sufficient for at least 3-5 years of livelihood.
Previous point is important because it makes somayaga NON MANDATORY for a vedic life. It is absolutely fine if one keeps doing the mandatory ones like ishti and chaturmasya etc. If someone is well to do and is qualified then may perform this HIGHLY CELEBRATED AND PRESTIGIOUS Somayaga.
Good points have already been mentioned by above answer.
The yajnas were not "banished", but they just gradually lost the gravitas they had in earlier times under the influence of Vedanta and Bhakti schools, which promoted other practices in substitution of performance of elaborate yajnas.
Besides, the bigger yajnas mentioned in the question are very expensive and require specialized knowledge on the part of the priests, as well as hundreds of other people with specialized skills.
There are only a few pockets of India where yajnas are still performed, but as per reports, even they have compromised on many details due to loss of tradition or just changes in sensibilities. For example, the version of somayajna prescribed most frequently in classical texts requires the sacrifice of many animals. But modern practitioners of yajnas often substitute actual killing by just pretending to strangle the animal and letting it free. Similarly, ashvamedha actually requires strangling the horse at the end. Although the ancient Brahmana texts already mention alternatives to animal sacrifice, such as using animals made of clay or flour, many purists insisted on actual animal killing.