We find an answer to this question in the work "Krishna Ashraya" of Sri Mahaprabhuji.
The chief aim in a man's life is turned towards obtaining such bliss (ananda) which will neither shortened nor will grow at any timer we have seen that the ananda we get in materialistic way, is not the real joy or happiness. It is joust Visayananda vedas have written about true bliss in a very beautiful way. We have to find out - sastras say - where true ananda is and in what quality. A measurement of ananda is mentioned therein.
Let us assume there is only one emperor on the whole world. Whose sovereignity is unlimited. Let us take him as unit of ananda which he enjoys. If you take hundred times of that ananda it will part only one unit of ananda it will celestial beauties called Apsaras. 100 units of Apsaras will form one unit of Gandharvas (Gandharvas are considered to be one step ahead in celestial being). Thus, the vedas went on counting the measurement of ananda in the world and ultimately reached Akshara Brahma. They said (vedas said) that if yoy count the ananda of Akshara brahma it is measurs the highest of all counts made that far.
But vedas have decided that what ever ananda can be measured in such a way has its own limitations. Because they have found out that even this highest ananda has the tendancy to increase or decrease. For some reason it does not maintain a steady measure of ananda.
They have discovered that the Purna Purushottama Paramatma, who resides in Akshara Brahma, whom we address as Bhagavan Sri Krishna, HIS ananda they could not measure by any standarad. They found it beyond the ability of counting. So it remained "aganita ananda" for vedas. Ultimately vedas have to admit : "Is it like this ? No! No~ not like it" and in the end it came to be known as "Naeti! Naeti".
Hence Sastras have described it as Purna Ananda. "Esa Vyasa" Upanishad has made a beautiful description about this phenomenon of our Prabhu. The sages said with upanished : HE is complete. As much as you try to draw ananda out of HIM, HE will always remain complete. Even if you attept to draw the whole contents of ananda HE will still be soon containing full ananada.
So we observe HE is the only deity whose ananda is perpetually full. And that deity is Purna purushottama Paramatma Bhagavan Sri Krishna.
Sri Mahaprabhuji has commanded :
Prakrutah Sakala Devah
Gani Tanandakam bruhat 1
Krishna eva gatir mama 2
All devas as under the influence of nature. Even the ananda of Akshara Brahma countable only Sri Hari is Purnananda. Hence Krishna alone is my salvation.
So since Sri Krishna alone is found to be having limitless ananda and is on top of the list, HE is treated as our ista (most cherished) parama (supreme) tatva (element). The word used very often in phylosophical parlance as tatva indicates towards a supreme power behind the whole universe.
Let us take an instance. Here we have a begger. He needs a rupee. Where would he go to get it ? It is quite evident that this begger would goto a person who is capable of donating the maximum.
So when we are seeking for some ananda out status is not better than this begger. So where would we go to obtain this ananda we are seeking. When all celestials have limited ananda how can you expect to draw the most out of a limited substance ?
Hence Sri Mahaprabhuji has asked us : Sri Krishna is an ocean of compelete bliss. Go and take HIS shelter. HE will some day grant that Purna Ananda to you. You will know then that the ananda you have received when once obtained will never ashrink. It is something which you as a bhakta can draw in this very life and with this mortal body. Then you will not require any Moksha (liberation of soul) to obtain that sublime, divine ananda. You do not even require another birth.
Hence Pusti system has considered Sri Krishna as "Parama Tatva".
Why take shelter of Sri Krishna ?
It is good to understand another reason why we should accept Sri Krishna as the "Supreme Element" (param tatva). In this world each person has to rely on some one or other to lead a life of independence. In childhood infancy they need the support of tather and mother. When grown and go to school and college you have to rely on your teachers for knowledge. While working you need the support of your employer. In home your relatives and family members stand by you when you grow old and walk with a cane you need the full help of your children. This more support one needs is expressed as "avalamban". We must try to understand the meaning of the word Avalambana in connection with our relegious life.
In these circumstance one will desire that it will be far better if you attempt to take the avalambana of a greatly capable person. There is a provert. It says : A capable enemy is far more preferable than a weak friend. The reason given is : The more the supporter is competent the less will be the difficulties you will have to face. But unfortunately in practical life one cannot keep contact with a truely competent person. Suitable to his desire.
Sometimes the children find even their parents are not quiet comptetent in their case. Hence we have to get on with watever little support we can secure in life. In seeking religions avalambana the intention behind should be : how much sukha and santi (happiness and peace) one can attain out of it of course behind this intention there is slight selfish motive also.
We know that we cannot find a fully competent person in the worldly matters who can provide us with happiness and peace by getting his shelter which is real and full. In these circumstances we have to look out for some extra terrestrial sources for such avalambana. For this many people seek the shelter of various celestial beings. (devis and devas). Then, why do the followers of Pusti system take the shelter (asraya) of Sri Krishna alone ? Is there not any one else to give us support ?
Sri Mahaprabhuji has given a very beautiful answer for this question ?