Before Sudebnik 1550 the peasant's didn't have the right to leave non state ( e. g. boyar's and church) land.
So, you insist that serfdom (krepostnoe pravo) was in Russia before 1550 and Sudebnik set the peasants free, don't you?
So we've got kind of abolishon of serfdom(krepostnoe pravo) in 1550? Are you sure about it?
Unfortunately after the death of Ivan IV there was no strong ruler in Russia for a long time so boyars could retake almost lost positions in the state which led to Sobornoe Ulozhenie(1649) – enforcing serfdom in Russia by the time when most European countries began its abolishment.
I will translate something from a Russian history book.
If you know Russian you can download this book
M.M. Shevchenko 'History of serfdom in Russia (krepostnichestvo)' Russia 1981
page 89-90
"To the end of 1570-s in the moment of almost total bankrupcy of economy of the country the competition (struggle) for labour (peasants) reached it's climax. Landlords tried everything not to loose their peasants. As a rule it was small and medium gentry (pomeschiki) who were loosers in this game. The lands of big landlords (boyars and especially monastery abbeys) were more economically stable. They were strong enough not only to keep their own peasants but they could attract a lot of peasants on a grand scale from other lands. And it was done without observing the "Yurjev Denj" and without paying proper payments to former lanlords.
So the government took measures in the two last decades of the 16-th century. The essence of these was prohibition of the right of the peasants to leave their masters and further development of serfdom (krepostnichestvo) of peasants."
So we see that boyars profited from the right of the peasants to change their landlords. So we can not blame them for serfdom in Russia.
(Sorry for poor translation. it is a nighttime and i am in a hurry