Then why did he also look bad BEFORE being traded?
He didn't which is why an NFL team gave up a #1 pick for him.
3.4 YPC last year in Cleveland. They gave up a 1st because they thought he was playing poorly since he was on the Browns. Turns out that wasn't the reason.
I still stand by my assertion that it wasn't that bad of a trade due to the fact they got him on a 3 year, $7M non-guaranteed contract. And if by some miracle he does turn into a great back, the Colts have the option to keep him another year (albeit at a top 10 RB salary).
I think NFL teams look a little deeper than YPC and I think you should as well.
ETA the following:
I'll add I think YPC is not a overly reliable tool due to a variety of factors such as blocking, usage(is the RB getting an inordinate amount of carries in short yardage vs a high amount on passing downs), supporting cast, competition that RB faces.
Also what's been pointed out with Richardson is he's not broke long runs. By itself not damning, see Eddie Georges career where his longest run after his rookie season was 40 yards, but it certainly would lead to a lower YPC.
Speaking of Eddie George I think most found him to a pretty solid NFL back,not a HOF back but pretty solid. His career HIGH YPC is 4.1 which he did twice and is the only two years of his career he broke 4 ypc. In part because he failed to break long runs.
Other notable HOF or soon to be HOF RB's who did not demonstrate high YPC early in their careers.
Tomlinson got 339 carries his rookie season and finished with a 3.6 YPC. In Trent's Browns career he got 298 carries for a 3.54 YPC. Not a lot of difference.
Going way back Walter Payton got just 196 carries his rookie season and for a 3.5 YPC.
Emmit Smith at 3.9 his rookie season which is almost half a yard more than Trent but not impressive either.
Jerome Bettis will make the HOF soon. His YPC his first year was outstanding. He then spent the next two seasons playing his way out of St. Louis with 3.2 and 3.5 YPC's but still the Steelers saw talent so in an unusual move for that franchise they traded for a player on another team. It was a great trade but Jerome would rush for less than 4 YPC in 7 out of his 10 seasons with Pittsburgh and would end his career with a 3.9 YPC average both for the Steelers and his career. Again that's better than Trent but not impressive.
Curtis Martin averaged 3.85 YPC after his first 6 seasons.
Lynch was 3.9 YPC after his first 4 seasons. Like Curtis and Emmit that's better than Trent but not by a lot if we are just looking at Trent's Browns career.
I am cherry picking stats here and you can find a whopping amount of RB's with YPC early in their careers. I'm just trying to illustrate is how many good RB's, many HOF RB's, had low YPC their initial seasons or over the course of their first few years in the league and it was not an indicator that they sucked.