Is that printed anywhere? Any reasoning for that?
Printed in his name everywhere, very known. His saying was something like "חסידות פאר א בחור איז פילעווען אין אביי ורבא".
Obvious reason would be that a yid should start with fulfilling first his basic obligations in yiddishkeit, and only thereafter he should start focusing on reaching a higher דרגא בעבודת השם. Not that bocherim didn't learn ספרי חסידות at all or didn't practice other stuff like mikva etc. But the focus would be first for the basics, therefore Tefillin on Chol hamoed which is a מנהג החסידים ע׳׳פ קבלה and against the עיקר הדין ע׳׳פ שו׳׳ע, he believed that buchrim should stick to עיקר הדין ע׳׳פ שו׳׳ע.