Migration is controversial so the compact is controversial.
If one of the basis of your political discourse is that immigration is the root of all social evils, painting the compact that as a threat that will force governments to accept immigrants against their will gives for some political points when you reject it1.
So, rejecting the compact would appeal to the governments who got elected on this premise, specially if the people are protesting the government. It allows the governments to move the debate towards the immigration issue and paint themselves as defenders of the national independence and sidestep other issues.
Of course, this is not restricted to governments, as political parties everywhere have mischaracterized the Compact in the same way in order to attack the government.
To use a quote taken from someone else's link
The Crown Law Office and the Ministry of Foreign Affairs and Trade had provided legal advice that confirmed the UN cooperation framework was neither legally binding nor constraining on the country setting its own migration policies, he[New Zeland Foreign Minister Winston Peters] said.
"The legal advice from Crown Law is not surprising but is important advice in debunking falsehoods or misguided perceptions being spread about the implications of this framework."
Of course, you could argue that those who signed it are trying to get points for signing a document that says "we are nice people who treat people nicely" but does not compromise them to any practical measure.