While everything you stated is technically correct (heroin essentially being a prodrug of morphine), I can see why people might choose heroin as their first choice but not morphine as their 2nd, and Ill explain why.
If someone is shooting these drugs, heroin and morphine, despite being so similar chemically, have a very different initial feel. Heroin crosses the blood brain barrier much quicker then morphine and thus has a much more pronounced "rush" then morphine does. So in this sense, the initial feel of heroin feels more similar to something like hydromorphone then it does morphine.
Also, morphine creates a much stronger histamine reaction (that pins and needles feeling) then heroin does, and many people do not like this feeling.
So, from this standpoint I can understand why morphine may not be someone's 2nd choice.-DG
nice post. i was just about to post something very similar but you beat me to it.