the "why so bright" part is simple: given same current, blue is noticeably brighter. A 5 mA current will make passable red, but flashlight-level blue. If you are old-school designer who got used to red and green LEDs, all your blues would be super bright. I still worry when I see 2K or a 5K resistor next to a LED: "Wait, that's too much resistance and too little current, user is not going to see it... hm, which color is it? Are you sure it's that sensitive? Maybe decrease resistance a bit just to make sure you don't have to redo the PCBs?"

Which also means that blue LEDs draw _less_ power than red and green ones, for the same brightness. Thinking about this, this could be a one reason to use them in battery-backed devices.

The "less expensive" part may be was true in the past, but is bogus today: a random search shows $0.01468 for red[0] and $0.01572 for blue[1] in 10K quantity. 0.1 cent is not going to affect your final cost, unless you are into super-cheap devices.

[0] https://www.digikey.com/en/products/detail/beking-optoelectr...

[1] https://www.digikey.com/en/products/detail/beking-optoelectr...