Hard to say.
First, lets be clear. HBS is a shot in the dark. Heck, half the people who even get in will tell you they don't know how they did. So, a denial without interview at HBS is hardly something to feel bad about. You have a lot of company in that boat. And, to be fair, Sloan is no cakewalk either. So before you beat yourself up too much, keep that in mind.
Now as far as your overall stats go, you are competitive - so there's no reason you shouldn't be a viable candidate (at least on paper). That said, there is a lot more to it than just your GPA and GMAT. Certainly, there are number of well qualified engineers with high GMAT scores such as your own.
Id be more concerned if you had applied to five in R1 and received no invites -- but two (one of which is virtually a guaranteed denial) isn't much to really draw any meaningful conclusions from. As someone once said to me "Dude, thats an N of 1."
Nevertheless, if you think your essays might not be up to par, have someone look at them. It can't hurt.