Microsoft’s David Cross stated at the 2008 RSA security conference that: “The reason we put UAC into the platform was ‘to annoy users’. I’m serious.'”
The intent of this statement is to encourage application vendors to eliminate as much as possible unnecessary privilege escalations and therefore allowing users to use not-Administrator Windows account without annoyances on their daily computing.
The unnecessary use of Administrator privileges, especially if it is needed just to run an already installed program, is what leaded many users to actually use the Administrator account for their daily computer tasks even if this wrong behavior has always made them vulnerable to malaware installation.