Short answer: probably because your actor has friction set to something above 0. Go to the physics tab of your actor, look under material, and there you can set your actor’s friction to 0.
Detailed answer: if we are talking realistic physics, then perhaps this conundrum doesn’t make sense to you. After all, in real life, its very unusual for anything to slowly come to a stop as it rolls down a wall, right?
But that’s just it. We are talking about video game physics. We borrow from realistic physics, of course, but often we break the traditional rules of physics.
In real life, when someone jumps off the ground, they can’t keep thrusting themselves forward the way they were on the ground; they have nothing to push off of. In video games, however, its not unusual to keep unrealistically applying forces even after leaving the ground. You can changes speeds and even direction while in midair, and that’s typically acceptable in many video games. However, if your actor has friction and is “running into” a wall, that constant force into the wall will often slow your actor to a stop because of friction. The constant force your actor applys towards the wall acts like a horizontal gravity.
So, typically the remedy for such an issue is to set the actor’s friction to 0.