I'd say mostly lack of knowledge about it, but sometimes people who are aware it exists don't like the fact that it allows arbitary queries, they think that only implementing specific methods provides more security - which can be achieved by limiting the odata access in any case, so that comes back to lack of knowledge.
Another factor may be that Microsoft used it so some sectors thought it was uncool and spent a long time building other stuff instead ..
I honestly can't say. It's quite easy to use and broadly supported. Even excel can directly pull data from oData Sources with a couple of clicks. If anyone else has some insights into the lack of uptake of oData I would love to hear them though.
