Hmmm...,presumably you mean why isn't working at an Urgent Care center more popular?
Although my EM colleagues are undoubtedly better positioned to answer this, I would imagine the repetition of the work would play a factor.
For example, Urgent Care centers may be less attractive as a "career" (rather than simply moonlighting on occasion) to EM physicians because they don't draw the very acutely ill as does a full-fledged ER. You will spend your shifts seeing patients with cholecystitis, diverticulitis, gastroenteritis, small cuts, lumps and bumps, chest pain, etc. If that's your thing, all the well...but if you're craving trauma, critically ill patients, etc. you don't want an Urgi Care Center job.
In addition, many people are drawn to FP and IM because of the continuity of care - an insignificant factor in Urgent Care practice (excepting the frequent flyers).
There are those who seek moonlighter hours at Urgi Care centers because the pay is pretty good, but I suspect is it less desirable for the reasons above (amongst others).