Is it because the rewards are too low for the work involved?

Yes, that should be part of the problem.

Or is it perhaps that people just don’t want to get involved in working to build and promote the platform?

I think many people are interested to promote STEEM.
However, most average users are having many tasks to handle in their real lifes, often a full-time job and/or children ... Considering this, requirements like (for example) writing weekly reports could have a deterrent effect, and just not everybody has enough time. That's just my guess though ... (but in my own case for sure having enough time would be the biggest problem).
Some patience is needed, and I guess the number of 'POD members' will increase over time.

Nevertheless, you are doing a good job, implementing some 'structures' (STEEM Greeters, the Diary Game, ...) to get and keep people involved in and active on STEEM.

Beside these useful and really needed fixed 'structures', in my eyes it is important to seek and support also these creative and free-thinking users/authors who write about many different topics which don't fit into any predefined, rigid categories. They are important to let the platform look as diverse and interesting as possible.
Therefore curators should seek actively for them and encourage them with their upvotes (I know that it's easier to scan certain tags ...). That's what I also do: seeking and upvoting great posts with nearly no pending reward.

Apart from that I would like to make two small, more technical suggestions:

• Users with less than 500 SP don't have the 'upvote slider' which means they always upvote with 100 % of their voting power. I think it would be really useful if all users, also the small ones, could decide themselves how much of their voting power they use per upvote.

• If I write a post in HTML and later edit it, SteemPeak shows the perfectly correct, original HTML code in its editor, while Steemit doesn't. The consequence is that I am editing nearly all my posts with SteemPeak only, which prevents me from switching completely to the Steemit interface.