Because a home that uses 11000 kWh/yr doesn't use 1.25 kW. During low power usage times (middle of the night) it uses substantially less, and at peak times it uses substantially more. And even these values will shift with time of year and weather. kWh/yr implies you are averaging energy use over a substantial period of time, whereas kW is ambiguous on its own. MJ/yr would work as well and have a similar order of magnitude, but it doesn't really have any advantage in its favor.