What limits Hubble’s ability to image planets around other stars, and why was it possible in only 1 case?

The only two things I can come up with would be the actual location of hubble inhibits other images beyond the others stars. Cost allot to reposition hubble and NASA is going out of business due to lack of funding. The other reason could be classified (top secret) until big brother declassifies the information. NASA can not publish of post on the web. After all NASA is a government agency.