It's mainly cost savings due to the simplicity of 25gbps ethernet vs 40gbps. This article has a good run-down: http://searchnetworking.techtarget.com/feature/25-Gigabit-Et...

sfp/sfp+/qsfp are just the connector types, and can terminate cables that are made of copper or fiber. This is a separate issue from what protocol (ethernet/infiniband/etc) is running over that cable.

For an example, take a look at Mellanox's cable page which lists a large number of combinations of connectors and cable types.

So in general, to call a random sfp cable/port an ethernet port would be confusing unless both parties already knew it was passing ethernet traffic.

http://www.mellanox.com/page/cables

I'm mostly mentioning this because it's something I just recently re-learned about.

The actual connector and cable are mostly independent of each other. The most common connector for an ethernet cable is RJ-45 (what you're referring to as "ethernet ports"). But it's just as possible to have SFP, coaxial, or USB even.