The real answer, WordPress hosting for the most part is the same as regular shared hosting. It is a marketing thing, if people feel they need wordpress hosting they are going to look for that.
Where it gets different is offering Managed WordPress which generally comes with a special server setup and/or tools specific for WP and/or more hand holding support.
I cant say for certain but i agree with this more hosts are "selling wordpress" what ive seen is "wordpress hosting" get offered with a free wordpress installation ( whoop ) and some demo content templates can be bought. I looks more like a offering a products and service that you already had access to on shared hosting in a more convenient and emphasised way.
Which works. I have seen that there are some differences in some packages but i doubt thats the driving force behind it.
There is no difference between both Wordpress and normal hosting. You can choose basic plan and install wordpress on it. Wordpress hosting is just you will get ready with wordpress installed Which cost you little bit than normal..