Because Kony Ealy is not viewed as a Right/weak side DE that they wanted and they already have a Left/Strong side DE group on the roster.
They over paid to trade up. Some will continue to argue other wise but Stephen admits it and Jerry admitted it multiple times in the third day PC.
With that said they also viewed 4 players that could help at weak/right side DE.
1. Clowney - that was never going to happen
2. Barr - He went before they could get him
3. Lawrence at a drop off from the above two.
4. Attaouchu but at a big drop off between the others including Lawrence.
They also though of taking Ryan Shazier and making the 3 tech (melton) and the LB (shazier) the focus of their pass rushing players instead of the 3 Tech and Right/weak side DE.
Shazier was taking by the steelers. The Cowboys were actually on the phone with Shaziers agent, Shazier's agent was actually writing a text out that the cowboys were going to take him...then the steelers took him one spot above the cowboys.
The next highest rated player on the board was Z.Martin.
They tried to get a good trade to move down and pick up an extra pick(s) and still get someone like Lawrence. The trade offers were not good enough.
So they decided that they were going to do what it took to trade up and get Lawrence because they had a strong feeling that Atlanta was going to take him at the top of the second.
They actually tried to trade back into the end of the first to take him but did not get a trade they liked.
They then decided to do it on the second day and got the trade. They admit to overpaying for the trade but they knew that he was not going to fall to them and they knew that he was a considerable step above the next guy they the thought could fill the role (Attaouchu).
That is why they made the trade.
Kony Ealy was NEVER going to be the Right/Weak side DE. They viewed him as a Left/Strong side DE who would also rotate in at 3 Tech DT.
Hope that answers your question.
So the trade may well be silly in your opinion, they did overpay by their own words...but taking Kony Ealy at 47 would be silly if they are looking to get someone to play in the departed D.Wares role because Ealy was not going to be that player.