Primarily because in the OT there are verses which denounce making images of God, etc yet when it comes to the NT, we have references to the Trinity.
So a Muslim thinks- hold on, the same God authored both revelations, how can they be so different? Why would God say it’s forbidden to make an image, it’s forbidden to worship anything other than God, then some centuries later we have the same God saying I will incarnate myself and die for everyone’s sins?
The revelation from God to Jesus is known as Injeel in Arabic and many Muslims, myself included believe that the canonical gospels we have today are not part of the original Injeel given to Jesus, hence why we think it’s been lost or corrupted.
I understand many Christian say that they don’t have to follow the Jewish law, because the sacrifice of Christ abrogates all of this, despite the fact that he was an observant Jew- again this is something that is totally at odds with Muslim thinking. When a Muslim wants to get closer to God, we follow the way of the Prophet Muhammad, even down to the way he cut his nails, walked on the road, etc.
Muslims aren’t telling Christians how to behave but it would be interesting if the popular question ‘What would Jesus do?’ was extended to all areas of life? If Jesus didn’t eat pork, wouldn’t Christians like to emulate that?
As you said that Peter effectively abrogated that law- Muslims would think well it’s Jesus that was sent by God not Peter?