1 of the reason why most ekiben are cold, is also because the smell from hot food is more strong, therefore, higher chance of the smell of the food to be transmitted within the cabin and might cause discomfort to other passengers if they can't stand a particular strong smell.
My hubby usually buys sushi set, so it is fine since it's usually eaten cold anyway.
For me, sometimes I have bento set which comes with either tomato fried rice or small serving of tomato spaghetti with some other ingredients like Chicken karaage, tamago-yaki, fish cake...etc. Not too bad when eaten cold though.
But yeah, things like Tonkatsu (fried breaded pork cutlet) is not really that nice when cold.