If we are just talking about the number of smart devices available for HomeKit thats because Apples strict security requirements limit HomeKits implementation.
Keep in mind Apple is in the business to make money. And to include that "Works with Apple HomeKit" logo on the box will likely cost a lot of money and time. So right out of the gate a company that just wants to get a product on the shelf are out.
But there are other reasons a company can't sell HomeKit ready devices. For example they need to be a MFi license which requires them to be a legal company in good standing.
Plus they need to abide by MFi license "rules" so for a HomeKit device for example Wifi and/or Bluetooth need to meet Wifi Alliance and/or SIG certifications.
Once you jump through a million more hopes your product needs to be tested by Apple and somewhere in there at least in the case of HomeKit you are authorized to use Apples Authentication Coprocessor. This is like the chip in the Lightning cable, without it you'll get an error "Accessory is not certified to work with HomeKit".
Plus with HomeKit the device needs to be powerful enough to deal with 3072 bit encryption keys and curve25519 for its end to end encryption.
That last part also stops certain manufacturers that want to collect data from their devices from implementing it.
Plus introducing a device onto a network is no different then having your network open. Is your smart crockpot making stew or collecting bank information and sending it off the network? Probably just making stew but without E2EE and no one to subvert it too you can't say for certain at least not easily.
So of the above list which would you be ok with? An illegal or business with poor standing? Hardware that isn't powerful enough for the high levels of encryption? Maybe one that just wants to collect that devices usage? I imagine with the big brand names none of those apply but there is nothing policing them either.
My only draw to home automation is Apple secure implantation. Without that I'll just as soon flip a light switch or turn off the crockpot myself. Although admittedly I actually see the value in a smart mouse trap
@steve knight lol.
HomeKit like other Apple "kits" is just the frame work though. If we are talking about how powerful (or lack thereof) the Home app is itself compared to similar all-in-one type apps...well thats just how Apple is. It works for most people but typically not as powerful as companies that specialize in that one thing. Personally I find the limited volume of devices and there associated cost to prevent me from usefully leverage IFTTT type automation.
I know I borderline a tinfoil hat wearer but we've already seen attacks from the internet of things and its only going to get worse. I'll use what I know is the most secure until its proven inadequate or something better comes along.