Why does Ayurveda place so much emphasis on factors like diet and lifestyle in treatment?
The individual, rather the patient, is an integral factor in the healing process and his/her full co-operation is essential to ensure the success of the treatment. Many a disease occurs when one goes against “nature” – to put it better, when one acts to the detriment of the needs of one’s body and mind on a regular basis. Thus even as a much abused car will eventually break down, a much abused body will also eventually succumb to the wear and tear of extreme tension. Even as a car requires daily and periodic maintenance, so does the body need to be looked after on a daily, monthly, seasonal and yearly basis. Even as the car cannot run on water or other liquids save what is specifically prescribed for the particular car model, so will the individual not sustain on an excessive and indiscriminate use of junk food as diet. Even as a car gets over-heated when used continuously without any rest, so does the body get over-heated and exhausted if not given periodic intervals of rest. Thus regulated diet and lifestyle are very important if one is to attain and maintain balance or harmony within and without.

Why is “pathya” (or regulated diet and lifestyle) given so much importance in Ayurveda – isn’t it very difficult to stick to an Ayurveda diet?
The importance of pathya has been explained in the previous answer. The concept of an “Ayurveda diet” is quite misleading. For, Ayurveda says that “there is nothing in this world that is not medicine (or food)”. Further Ayurveda says “there is nothing in this world that is absolutely good or absolutely bad”. Thus the important thing is to become fully aware of one’s own needs in the background of variables like place of residence, climate, foodstuffs natural to the particular place, one’s digestive potential etc., and to act accordingly. Thus what may be good for one person may not be good for another person in the context of all the involved variables. Another important point in this context is the rule that “one must exercise moderation in everything one eats or does” – otherwise even nectar can turn poisonous and thus bely its very purpose.
However for the purpose of easy understanding, and based on some general rules, certain foodstuffs are normally classified as difficult to digest, and since most people do not have a very good digestive system, these foodstuffs are normally avoided. But it must be emphasized, that if an individual has a very good digestive system and if such a foodstuff can be beneficial to him/her, then it will certainly be advised.
Thus there is no such thing as “Ayurveda diet” and very often most of the food we eat in a normal day is quite alright, and only small changes need to be made where necessary. For a patient, till the treatment is over or the disease is cured, the “pathya” will be applied more strictly, for here, the importance is given to the “treatment/medicines” rather than the food. The patient gets the greater part of his/her nutritional requirements from the treatment/medicines and hence it is important that the digestive system keeps its focus on the medicines and is not distracted by unnecessary food (which is mostly difficult to digest).
But will I get the necessary nutrition from the food you prescribe? To me, it seems rather insufficient in terms of proteins and vitamins.
As mentioned above, you will certainly get all the desired nutrition from the prescribed food, and more importantly from the prescribed medicines. The food may seem insufficient by itself but in combination with the treatment and medicines, your body is definitely being well cared for.

Does Ayurveda say NO to non-vegetarian food and alcohol?
Ayurveda does not say NO to non-vegetarian food and alcohol. The nutritional and therapeutic qualities of these substances are described in great detail in the Ayurveda texts, and in certain ailments, they are prescribed as medicine. Thus it is quite apparent that Ayurveda is not against non-vegetarian food and alcohol.
However as described earlier, these substances generally fall in the category of “difficult to digest” foodstuffs and hence they are not normally recommended, especially during treatment.
Further, at the mental level, Ayurveda describes 3 states of mind namely sattva or the pure state, rajas or the pro-active state and tamas or the indolent state. Ayurveda also says that the most subtle essence of the ingested food after digestion, goes on to form the mind. Generally, most non-vegetarian food and alcohol fall into the category of rajas and tamas and hence regular/excessive intake of these substances can lead to the affection of/reduction in the sattva or pure state of mind which will consequently lead to derangement of the thinking process, difficulty in comprehension, increased tension, emotional outbursts, restlessness, excessive sleep etc. based on the level to which one is affected. Since, according to Ayurveda, the mind and body are closely related and all diseases fall into the category of psycho-somatic, the body will also consequently be affected.
Hence as a general rule, Ayurveda advocates vegetarianism and abstinence from alcohol. If at all one would like to indulge one’s taste buds, these substances can be consumed “in moderation” on rare occasions, again keeping in mind one’s digestive capacity.