But I guess my question (or the part I'm more interested in hearing) is WHAT makes PowerPoint or even Keynote the only viable solutions.
Depends on your needs, how much control you want over your presentation and what your expectations are.
In my world, (live corporate/association events) we need something that is versatile, extensively customizable and reliable. Animation capabilities are of great value (most clients seem to want an After Effects show without the budget or time or comminment ;-); the ability to *reliably* adapt to a variety of aspect ratios and resolutions and then sling around a lot of pixels in them is an absolute must; the ability to string several decks together and *seamlessly* jump around them is essential (something Keynote still needs to work on outside of its terrible, awful presenter mode); and THEN the ability to quickly reformat *everything* because the CEO just now finally saw the look and hates it. And of course there's the aesthetics of how an app handles transitions, animations, videos, etc. (here Keynote excels). Those are just some of the main requirements we're looking to fill. (I won't even get into things like satisfying all of those needs and then having to deliver to the client a version they can use in the future -- usually a Powerpoint file).
Now, don't get me wrong: neither Powerpoint not Keynote satisfies all these requirements all the time -- we often have to jump production from one to the other as needs dictate -- and both can be extremely frustrating to work with. But for our needs, they're the only two real options. (Many times we'll go to Director if the animation really needs to get heavy, but that has tons of downsides as well).
We looked at Prezi, and it's nice as far as it goes -- but it only does go so far. (Plus, spinning/zooming around all the time on a 60-foot screen is going to make an audience puke ;-). I hadn't seen slide rocket, so when they get back around to selling it (!) I'll take a look (from their website, though, it seems more aimed at sales presentations and analytics and stuff, which is less appropriate for our end of things).
But yeah, right now Keynote and Powerpoint are the staples. I'd *love* to see something else with those features an capabilities. But nothing so far … sigh.