As I recall, applauding someone's reinstatement was deemed not appropriate since the person would not need to be reinstated if they had not first committed a sin which they were unrepentant for doing which resulted in them becoming disfellowshipped.
'Returning to Jehovah' is an automatic expectation and duty which WT considers the reinstated person should not be praised for doing.
The other reason I recall is so as to not elevate the importance of any individual. Similar to not celebrating someone's birthday, no human is to be elevated and praised above any other human. (Right!! Are not Pioneers, Elders, COs etc elevated above R&F JWs?)
While the Bible provides an example in the parable of the Prodigal son, the father forgave the wayward son and was excited with the sons return after a long absence, holding a feast in the son's honor, even as the father had not yet established the reason for the son's return.
WT disregards various bible examples of kindness, tolerance and loving behaviour when convenient to WT's agenda.