Well, you may as well ask why Janeway didn't make herself an Admiral. They really weren't in Starfleet's normal operations sphere anymore so there really wasn't much point to promoting Kim. He was already part of the Senior Officers group in the first place (which for someone out of the Academy, was a pretty big thing).
And the reason why modern Navies have a "Get promoted or retire" mentality is because in Modern Navies room for officers is limited because said Armed Forces are limited to Earth. They only have so many ships and so many people going in and out.
With Starfleet, they have all that vastness of space and new ships being built all the time so there's no need for that limitation anymore because there's always more room. So there's no real problem with not getting promoted once every few years.