Theres a few reasons RT has not done well:
1) MS stores only. MS stores are not exactly popular places, so it is highly unlikely that people are going to change their routine just to get a new product. Besides, if the rest of the MS stores are anything like the one out my way (Cincinnati) then they have a looong way to go before they are going to be useful. They need some intelligent life there that is capable of understanding a user's wants and then selling it to them rather than going by the book and saying 'well these devices come in different colors!'. Got to hand it to them though... they are enthusiastic. Now that there is a little more general availability I think it will help... but there are other issues as well.
2) No desktop apps other than Office. Seriously, why on earth is the MS Store limited to Metro apps? Open up the platform to allow for metro AND desktop apps. RT has a perfectly functional desktop environment, open it up for development! If there were desktop apps in the MS Store on Win8 I would definitely throw some money that direction because desktop apps do not consume your full attention like a Metro app does. As it is now, most of the Metro programs are just flash or phone ports that I would typically run in the background while doing something else. It is not that I do not like Metro, it is a fine idea, but the desktop is often better for a lot of things, and if it is there then it ought to be usable.
3) It's a Tablet. I do not know a single person who has actuially enjoyed their tablet purchase. Be it an iPad, or an Android, or winRT, tablets up to this point are just cute little devices that are good as a supplement to a PC or laptop. You can browse the web... so long as you don't need flash, or the web based programs your are running are not too big. You can listen to music... but it is too big. You can watch movies... but the TV is generally right there anyways. You can play games... so long as they are just glorified flash games... but without flash. Generally speaking, most things done on a tablet are simply better done on a laptop or a phone in the first place, and even Android tablets are not exactly big sellers. The only reason the iPad sells well is because it is trendy. I have to use them on a regular basis, and they are frankly not useful except in rare and niche cases... and even then a different device would often suit the job better. I think this changes with the Surface Pro, and will really change with later generations of it as it does not have the limitations of most tablets (granted at the sacrifice of battery life and weight).
4) ARM. Nothing against ARM itself. ARM is in my phone, my car, my kitchen appliances, my stereo, my TV, my HVAC, etc. ARM is the chip that makes life work. It is a beautiful low power limited instruction chip that is extremely stable because of it's simplicity, and so it works its way into all sorts of things. But once you start talking about something more complicated than a phone then ARM starts to choke. ARM is simply not efficient. It is extremely low power, but it is not efficient with what power it uses, and so when you blow an ARM chip up with all of the extensions you need to run a computer then it's advantages over x86 dry up very quickly. The point is that iPad and Android users don't mind the limitations because it is not a huge limitation on those platforms. Windows users however tend to be heavier users. We keep lots of things open, we let programs chew on things in the background while we do something else, we play games while watching movies. Even normal Windows users are heavy users compared to those who are accustomed to more mobile platforms, and so then when we look at the RT we are instantly annoyed that we simply cannot do as much at the same time from both a software, and a hardware perspective. Maybe ARM gets insanely better over time? I'm not sure, but as it sits now RT is simply not a particularly useful device for multimedia, or work, much less both at the same time. It is a beautiful first attempt... but it is missing something.
5) Other than the Xbox product line, MS is not known for being a hardware company. Their Keyboards and mice are OK, but not great. Their webcams are OK but a little expensive for what you get. The Xbox itself was plagued with the RROD, and even though that problem is not such a big deal anymore, people are still a bit scared of it. The Zune itself was actually a great little device, but MS did not support it well with advertising or updates (or a decent name that could attach some meaning as to what it is). Windows Mobile always had lack luster support (though it could do anything), and it has taken a few years for WP to really seem to get the full backing of MS behind it for a dev and support cycle. Even the original Xbox had a very short life cycle and was replaced fairly quickly (4 years). So with all of that history and baggage it is perfectly understandable that MS would have a hard time convincing people that it is going to have good hardware that it will stand behind for any length of time.
6) The general public does not know who MS is. Living in the tech world it is hard to believe, but the general public has no clue that the company that makes Windows is also the same company that makes Xbox or Office. MS has done a phenominal job at making its sucessful products ubiquitous and household name products, but it has done a terrible job at the corporate level at letting people know that they make other stuff. I think that is what I am most enjoying about MS with their changes over this last year or so, they are making a conscious effort to unify their products as much as possible. When you look a Win8/RT, WP7/8, and the Xbox360 there are some definite cues that these are all products of the same company rather than individual products. Same goes for their redesign of the services that use to go under the 'Live' brand (email, skydrive, etc.). MS is doing an amazing job at bringing it all together so that people recognize a parent company rather than a product that has some form of preloaded software on it (that is REALLY difficult for a large company with a lot of very separate product divisions). I think that this branding and integration will feel a lot more complete when Windows Blue comes around later