IoT is happening already all around us - but not so successfully in our homes. Yeah - at the radio link end, there's lots of different options that don't work together (and in many cases could NEVER work together because they do totally different things) but hey this is why people make gateways! Also there's a lot of movement at the API end for connecting together "Silo" products... not that I think that's the best way forward, but it's something.
anyway... rf options for IoT things...
wifi - really good option for stuff to go on a home area network. it's a solid and reliable and well supported transport, and everyone pretty much has it already. And when you use it, you can use any LAN or internet resource you want.... its weakness is power usage but SOCs that can get on, do their thing and get back off very quickly can still run for years off a coin cell if needed... they keep adding stuff to the spec (like wifi-direct which would have been a really nice thing to see people take up - allows BT style advertising of what is available on a network before joining) but people seem happy with the basics, and stuff like this hasn't ever caught on. Also connecting a wifi IoT device to a home network can be a pain, current best practice seems to be the direct login and configure dance, which requires the person using it at home to know the wifi router setup, etc. Needs a special phone app, needs to spend at least a few minutes per device...
zigbee - was an old attempt to make a network that's good for home automation, but in practice it's pretty crappy, and quite a pain in the bum to setup and maintain. They totally missed the idea of a simple auto configuring/minimal config network when they made this, because it was designed by geeks for geeks. If you're a geek and want to build yourself a simple collection of non-moving sensors and devices around your house, I think zigbee is an OK choice... you want to build a product to sell? nope.
Thread - looks like what you'd do if you wanted to make a better version of zigbee.. even uses the same 802.15.4 standard for the PHY - unfortunately the licensing costs they bunged onto thread will mean the community of developers they need for thread to become a serious consideration will not happen. Which is a real shame, because I get the feeling if it could make headway and get used by lots of developers it could become the simple low power HAN where multiple manufacturers devices could work with a single gateway.
6lowpan - seemed like a nice idea, but it also seems that nobody ever cared to use it for anything...
All other 802.15.4 protocol options - I'm sure they all have a purpose and relative strengths/weaknesses but really they all just seem too similar yet different. if you make a standalone system of things that can use one of them, then go ahead, but your system will always need its own gateway if you want to get out of the local network.
bluetooth - has only ever really been about connecting stuff to phones and computers, even though it theoretically can do a lot more, nobody really uses those aspects for anything, and therefore those aspects of it are really not supported in practice. BT5 looks promising, and I heard rumours about what 6 will have that sound really interesting - but basically they're trying to extend towards and compete with wifi... I expect people will not use this much because wifi already works for what they use it for, dual BT/wifi chips already exist, and BT will stay a personal area network solution..
ANT - made some headway in sports stuff, but seems to be at a bit of a dead end. Not sure why you wouldn't just use BTLE.
LoRaWAN.. I can see that getting very big, if the LoRa alliance get their current project to implement position determination in the base stations working (sooooooooon apparently)
Sigfox... I see a lot of investment in infrastructure here in Australia, and it seems to be rolling out over a lot of the world, but people are rightfully wary about investing into a tech where only one company owns the data transport forever... I dunno. Wouldn't buy shares in this, personally...
NB-IOT (LTE variant) - a serious competitor to LoRaWAN and likely a sigfox killer if the standard can get written well and fast and implemented in silicon and supported by cell tower manufacturers in time..
3G, LTE - works for plenty of use cases (some cars nowdays have multiple cellular modems in them for telemetry purposes already) though costs a lot to power and maintain subscription don't work for a lot of things people want to do.
I think the big problems with "IoT" as a buzzword are:
1) the fact that everyone seems to think it's just one singular thing that you can roll out over the world in every use case.
2) the utter lack of imagination and poor understanding of user requirements that many IoT things have had so far (and the really greedy subscription models that business people these days want to pursue)