Why are not all IMAX theaters domed?

If you really want value for your IMAX experience, you will only go to see films that have been shot using IMAX 70 MM film or the digital equivalent and that project on film, not digitally. Most Hollywood films do not fit all of these components but the popularity of the concept gets butts in seats (at a higher price point) for something they think is higher quality but likely isn't.

To answer your title question, no, not all IMAX theaters are domed... even true IMAX has more flat-style screens... in fact, the original IMAX screens were only slightly curved. A special setup had to be made for the domed screens you've seen.

Having a giant screen doesn't really mean anything if they're still projecting a low-quality image. According to Wikipedia, many current IMAX digital projectors are only projecting at 2K (which is crazy since a lot of digital non-IMAX theaters are projecting 4K!):

Because 70mm film and projectors are costly and difficult to mass produce, IMAX debuted a digital projection system in 2008. It uses two projectors that can present either 2D or 3D content in DCI or IMAX Digital Format (IDF) (which in itself is a superset of DCI). As of 2012, IDF uses 2K-resolution Christie projectors with Texas Instruments' Digital Light Processing (DLP) technology alongside parts of IMAX's proprietary formats. The two 2K images are projected over each other to make the image brighter.

The digital installations have caused some controversy, as many theaters have branded their screens as IMAX after merely retrofitting standard auditoriums with IMAX digital projectors. The screen sizes in these auditoriums are much smaller than those in the purpose-built auditoriums of the original 15/70 IMAX format.

This 2K resolution is about half what the film resolution is:

Another disadvantage is the much lower resolution of digital IMAX compared to traditional IMAX film, which is estimated to be up to 8,700 lines of vertical resolution on the camera negative and 4,500 on a release print.

As of 2013, some theaters have upgraded to 4K projection, which is much better but you'll have to ask the theater which they use, as it's not standardized:

In late April 2012, IMAX began testing a new 4K laser-projection system, based on patents licensed from Eastman Kodak. Like the film and digital systems, it uses two projectors but it has been said to improve over the smaller digital screens by retaining the traditional IMAX aspect ratio and to allow films to be shown on screens 36 m (120 feet) wide or more. The laser projector will be available to exhibitors in the fourth quarter of 2013.

Is there any hope for the future?

According to the Wikipedia article, yes...

In May 2015, it was announced that Marvel Studios's two-part film, Avengers: Infinity War, will be filmed entirely in IMAX, the first Hollywood feature film to do so, using a modified version of Arri's Alexa 65 digital camera. The camera is set to be used first to film select sequences in another Marvel production, 2016's Captain America: Civil War.

Other films have select sequences that were shot on IMAX, including the upcoming Star Wars: The Force Awakens.

I've skimmed over a ton of info in the Wikipedia article, which is really all you need but this quick and dirty overview should give people the basic concepts.

First of all, IMAX is an acronym for "Image MAXimum". And from what I have seen and read, IMAX theaters are only domed for planetariums, which is why they are domes at the Tech Museum and at the U.S. Space and Rocket Center. The purpose for IMAX to even exist is that it has the capacity to record and display images of far greater size and resolution than a normal camera could. I assume that they were just using the IMAX theater to trick people into thinking that there was something different about it. However, another type of IMAX will shoot films at 48 fps, so the movie may have appeared more smooth. I am not an IMAX specialist so I do not know the specifics as to why they would show Ant-Man in IMAX, but I hope that I have cleared up some of your questions.

Domed IMAX theaters are OMNIMAX theaters. Most Imax films aren't Omnimax films, so there's no need for Omnimax theaters.

Proper Omnimax films are filmed with a special camera with a fish-eye lens. Projection is done using a similar fish-eye lens, on a domed screen. This makes for a much more immersive experience.

Not all Imax films are Omnimax films. In fact, most are not. Regular Imax films can be projected on an Omnimax screen, but will be skewed on the edges, since they weren't filmed in Omnimax.

With so few Imax films being Omnimax, there's no great demand for Omnimax theaters, which I can imagine to be more expensive than regular Imax theaters.

Omnimax is a niche within the niche that is Imax. It's a miracle they're still around.

The Omnimax concept was created for the San Diego Science Centre who wished to supplement their dome shaped planetarium and did not wish to incur the cost of building a stand alone IMAX theatre. This concept caught on with a number of science centres and it became a point of discussion whether to install an IMAX or Omimax theatre.