Because buprenorphine is a very powerful agonist (well, partial agonist) at the mu-opioid receptor. This means that, at high enough doses (usually 3-4mg, higher for some people) it has a blocking effect. This means that buprenorphine, just like methadone (again dosing is important here) blocks other opiates from the receptor. So if you're on bupe maint. you can't (in theory) get high off your original DOC. Buprenorphine also has a "ceiling effect"...somewhere around 24-32 mg...at which point dosing higher won't get you any higher.
In regards to buprenorphine being used as pain management...well, it is an opiate. I fail to see the difference between using buprenorphine for pain management vs. using other opiates. In many respects, buprenorphine is a better choice for pain management because it has a long half life and requires much lower doses due to its high potency.
In theory, any opioid/opiate could be used for Opioid Replacement Therapy (
ORT). Suboxone (buprenorphine + naloxone) and methadone are the only approved drugs for ORT in many countries. The idea being ORT (be it with buprenorphine, methadone, dihydrocodeine,
LAAM, or even, in some rare cases, pharmaceutical grade
heroin) is that opiate users are going to use opiates no matter what. Replacing their habit with a pharmaceutical grade drug in a medical treatment setting vs. a drug they buy on the street is the goal here.