I think it is primarily a branding thing that makes the school more attractive to a larger base of people - particularly foreign students I am told.  In recent years it’s become the “in” thing to do for several Colleges that qualify through degree offerings etc. 
For example St Joseph’s College which still has a Brooklyn Campus but is headquartered in Patchogue LI became a University.  Similarly, Molloy College in Rockville Centre LI recently became  a University.
St Joe’s had always been known for its very strong Teacher Education program, Molloy for its nursing program and Manhattan College primarily for its Engineering School. 
I tend to think of school size as more of a discriminator between College and University.  Manhattan is a relatively small school - I think on the order of 5000 students or so.
So for me personally,  Manhattan College which I graduated from in 1970 (I’m an old guy now) will always be Manhattan College - I still wear my Manhattan College ring and am still very proud of the school and the engineering business networking my fellow graduates fostered throughout my business career - we used to be called the “poor man’s Notre Dame of the East” amongst my colleagues.  I was also a “day hop” and did not board like most students do today.  I lived at home and worked to pay my tuition. 
As a Manhattan College guy I wish Msnhattan University all the best but it’s going to take a while to get used to the new name - the old name worked for 175 years or so but as I always told my kids “life is a continual series of adjustments”