The sensors use Bluetooth LE because it's more cost effective hardware than WiFi and it uses a great deal less power, so the batteries last a lot longer (in many cases 2 years) and the sample rate of the sensor can be a lot higher than if it used WiFi. A lot of sensors with on-board WiFi will sample once every 15 minutes instead of once per minute (like SensorPush), and their batteries die after a couple of months instead of a year or more -- which is a real annoyance when you're measuring hard-to-reach places like a crawlspace under a house.
So with a Gateway, you can have a larger array of sensors pushing to the cloud for remote monitoring for a lot less cost and also using a lot less power than if they were WiFi. Plus, the SensorPush cloud service is included in the purchase price, so there's not monthly fee for the Gateway.
We may eventually move in the direction of wifi once it's possible to have more efficient systems.
