Why do some rental sit on the market?

[Aasim T Bickiya]

I want to invest in class A and Class B rental in the suburbs of Birmingham such as Helena and Calera. But as I was browsing Zillow I noted some properties that were put up for rent were sitting vacant for months on end. The properties also looked pretty great too from the inside and the PM company was giving out deals such first free month or gift card. . Why aren’t these properties in class A and B areas renting out? Kinda scares me that my property won’t rent out.