Assuming serious question (well, why not), nothing in particular other than the cost. It's one of those things that you expect never to need and "domestic" sprinklers are a fire-suppression thing, rather than a fire-fighting thing so they're not going to save your house from burning down. The idea is to give you a few additional minutes to evacuate and hopefully to limit the damage done before the fire brigade arrives.
Wales led the way in making sprinklers mandatory in public buildings some years ago, and they became mandatory in domestic buildings the very same month we gained our planning permission.
Our project is fairly small with a total build cost (without land) of somewhere around £180,000 so the £2,500 basic cost of the sprinklers forms less than 1.5% of the total cost, but the smaller the project the greater the proportion.
Sprinklers need a 32mm water main which will be an additional cost even if you are re-building as we are - we had a ½" water supply and most houses built in the last 20 years will only have 20 or 25mm. If said water main cannot provide the pressures and flows necessary for "direct" operation, then you will either need a suction pump (in our case that would add £1,500 to the cost) or you will need a pump and a storage tank for which we were quoted an additional £600 (possibly less, subject to survey) on top of the cost of the pump. Oh, and you have to have specialist electrics installed for the pump (it can't just be run from the ring circuit powering the dishwasher) and need a large amount of floor space capable of holding a tonne and a half of water.
On a purely practical level, the heads have a limited range so tend to be installed slap bang in the middle of most rooms, exactly the spot where you would normally put a light fitting. More than that, a standard 6" pendant cannot be installed closer than 30cm to the head and larger, more showy, dangly fittings should be even further away. Try explaining that to the builder struggling to cut the hole in the correct place in your ceiling boards while also trying to place the lighting cable.
The only variable is the number of sprinkler heads and connecting pipe required. The cost of installing the water main, the cost of the pump and the cost of the tank are (pretty much) equal for most "normal" sized houses and in our case we could possibly be looking at a bill of £4,500 plus electrics plus losing a space the "size of a filing cabinet" (according to our installer). This amounts to 2.5% of our build cost, or to put it another way, it's about the amount of money we've set aside to use for kitchen and bathrooms.
Oh, and there's an ongoing cost of maintenance - the system has to be drained and inspected regularly. By a qualified installer. In the case of a tank system that also means chucking away 1½m³ of water, and although we are unmetered with our ½" water connection, the 32mm connection comes with a mandatory meter. Somehow, they propose that water used for fighting fires is not metered. I have no idea how they will be able to separate that out!
It's just "one more thing". Self-build can still be better value for money than buying from a spec. builder and having the freedom to design the internal layout is brilliant (exactly which one of our children is going to be happy with the "box room" typically found in those properties, when siblings have larger rooms?) but costs like this - which spec. builders can often get around or at least reduce - are making it less so.
Yes, fibre is definitely the way to go, but even the capped cost is a lot higher than the cost of connecting Copper in most areas.