Why do «بِإِبْرَاهِيمَ» and «بِنُوحٍ» have different endings after bi-?

Diptote Causes — Foreign Origin and Its Conditions

• One is a noun and the other a verb
• One is a diptote, the other fully declinable
• Bi- governs only Ibrāhīm
• Nūḥ is indeclinable