Well, I could just say because Game Freak wanted it that way, but I'll throw my theory out here.
First of all, take a look at their sprites.
As you can see, Trapinch looks like it could really put the hurt on another Pokemon with those jaws. Vibrava loses those impressive jaws through evolution, and looks a little bit less dangerous and/or strong than Trapinch, which makes sense why its attack would lower.
Also, the Pokedex entries makes sense too.
TRAPINCH’s nest is a sloped, bowl-like pit dug in sand. This POKéMON patiently waits for prey to tumble down the pit. Its giant jaws have enough strength to crush even boulders
To make prey faint, VIBRAVA generates ultrasonic waves by vigorously making its two wings vibrate. This POKéMON’s ultrasonic waves are so powerful, they can bring on headaches in people.
~ Pokemon Ruby
So to simplify it, Trapinch crushes boulders and tears stuff apart, while Vibrava generates ultrasonic waves with its wings.
Also, there are plenty of examples of other Pokemon losing a few points in one stat through evolution such as Weedle ---> Kakuna (which makes sense too).
Hope this helps :)