why bolt is made for networks less than or equal to 2.5 ghz?please explain the reason behind it
Bolt uses the ESP8266 chip which is made by espressif systems. The ESP8266 chip uses 2.4 Ghz Band which is a standard ISM band approved by IEEE used in wireless networks especially Wifi Routers.
2.4 GHz is used as you get a longer range and a smaller antenna (cost effective & smaller design) is enough.
As also mentioned in the training, Bolt module uses on a 2.4 GHz a band. This is because, most of the Wi-Fi devices run on this band. The newer devices which support Wi-Fi with higher frequency band support 2.4 GHz band as well. In my opinion this is just to ensure that every device of yours can connect to your Bolt Wi-Fi module.
Hope it helps!
There is no tumbling thought over what should be the ideal frequency amongst the two frequencies. It all depends on the user requirements.
If you want a better and a longer range for your devices, use 2.4 GHz.