>

Lots of reasons, drug abuse is often a reason they are in their current situation. Meth is really big out here in Los Angeles and is an extremely addictive drug. Use of drugs in temp housing creates HUGE liability risks. 
The approach you are describing is essentially what Los Angeles has been doing and it has made the problem worse. 
Again, you are right about more temporary housing. Per the Boise case, camping laws are difficult to enforce if the city can't demonstrate that temporary housing was available and offered. 
This is a complex issue that needs a multi faceted approach. If you ever see an article saying that it only requires "this easy solution", they have no fucking clue what they are talking about.
What is the long term solution? >

Giving people short term housing is proven to get people out of homelessness. >

Alot more is needed than that, mental health and addiction treatment, often times (not all the time) housing will be declined due to prohibition against doing drugs/curfews, etc. 
Cal CARES Court which will hopefully be implemented should help with this and also enforcement compelling individuals to get treatment they need. >

Just let them do drugs, jeez, why is it any of our business. They'll get sober when they are ready and no one can force it to happen before then.