na veda-yajnadhyayanair na danair
na ca kriyabhir na tapobhir ugraih
evam-rupah sakya aham nr-loke
drastum tvad anyena kuru-pravira

"O best of the Kuru warriors, no one before you has ever seen this universal form of Mine, for neither by studying the Vedas, nor by performing sacrifices, nor by charity, nor by pious activities, nor by severe penances can I be seen in this form in the material world."

Kuru-pravira = best warrior in Kuru Dynasty [Arjuna], tvad anyena = others apart from you [Arjuna], aham = I [Sri Krishna], evam-rupa = with such image, drastum = seen, na sakya = not possible. Why so is explained in the First half of sloka. Excepting Arjuna none else could have seen the Lord with entire Universe as His body. Na Veda = not by listening to Vedas, na yajnadhyaya = not by performing yagnas, na danair = nor by bountiful gifts, na ca kriya bhi = and not even by homa, na tapobhi ugrai = not even by intense meditation, the Lord could be seen. Therefore, by tapas, by homa, by yagna, by dhana, by practicing Vedas, by listening to Vedas, or by any action, the Lord's this image of Viswaroopam could not be seen by anyone. Only Arjuna was fortunate to view. What was Arjuna's doubt and how the Lord replied him are to be seen. When Viswaroopam was shown, Arjuna saw along with him, others viewing the Viswaroopam. The Lord earlier blessed Arjuna with special eyes to view the Viswaroopam. Also we saw earlier in another sloka that seeing the Viswaroopam, Duryodana and others were scared; while Bhishma, Drona and others respected with folded hands. But the Lord did not bless them with special vision. Sanjaya got the Divine vision by the blessings of sage Vyasa. Then without such Divine sanction, how Duryodana and Karna and others were able to see Viswaroopam? Arjuna's doubt was that when others also had seen Viswaroopam, how the Lord was stating that none other than Arjuna had seen Viswaroopam? Are there other methods to view Viswaroopam? These were the doubts in Arjuna. Many had seen the Viswaroopam and for all of them the Lord did not grant the Divine eyes. They saw but not in the proper way to see the Lord. When one is seen, knowing all about one, then one is said to have been seen. But Duryodana and others did not see the Lord in that proper way. The Lord showed Arjuna so that he got a determination. By seeing Viswaroopam, Arjuna realized the greatness of the Lord. Seeing alone by Arjuna might not induce the same full realization, as when others like Bhishma also saw and realized His greatness. When wicked persons also saw and got scared, Arjuna's realization got strengthened further. That would make Arjuna to resolve that he should also be in the group of noble ones and respect the Lord. To induce this determination, the Lord without actually showing the Viswaroopam to others, made Arjuna to realize the feelings of fear and respect in others. The feeling of love and regard in Bhishma and others, was shown to Arjuna. Only Arjuna got the Divine eyes from the Lord to actually view Viswaroopam. Now, the Lord says that the Viswaroopam could not be seen by yagnas, Homa, etc. Here we may get another doubt. In Vedas it is told that noble ones realize the Lord, by yagna, by gifts [dhana], by tapas [meditation], by fasting, etc. But the Lord states here the opposite and says that all these would not enable to see Him. Here we have to add one qualification. All these yagna, tapas, etc., as such would not be able to show Him, if done without Bhakti or devotion. Sans bhakti, mere dhana can not show the Lord. To see the Lord, therefore, all these actions should be associated with Bhakti. This is what we have to understand from this sloka. Without expecting any rewards, pure Bhakti is needed. Devotion or love, only to Him, unalloyed by any selfish interests would help in seeing Him.