While I'd argue that you should always count both your winners and losers regardless of whether you have a trump, I think this advice stems from the reduced control you have over the game in a no trump bid. Since you have no trump suit that allows you to regain control, there is a greater ability for your opponents to exploit your losers.
While personally, I have never heard this advice quite as you tell it (I've been away from bridge a while) when your are considering a trump bid you can use ruffing/cross-ruffing to avoid losers. In no-trump your only way avoiding a loser is establishing a suit; which you typically won't be able to do. (If you could count on establishing a suit you likely would have chosen that suit as a trump.)
The other way of thinking about this is that in a no-trump game the tricks will be spread pretty evenly among the suits. If you have Kxx, you will likely win one of three tricks. In a trump game, more tricks will end up being played with one or more trumps played: there is a greater possibility that only two tricks with that suit will be played (or played without being trumped by you or your partner). It is more feasible that you will only have one trick lost in that suit even though that is considered a "two loser" suit.
Stoppers are probably even more important to consider when making a no-trump bid than losers, but that's somewhat similar in considering your losers. A difference being that xxx (or xxxx) is considered only 3 losers, but since there are no stoppers you could easily lose four or more.