It may be because some NiCad batteries fail sorted. If one cell is shorted, which is fairly common, the charging circuit would be working overtime to bring the charge up to what it thinks it should be, and it never makes it. I suspect that could have a long-term adverse effect on the charging circuit. I don't know this is the primary reason, just speculation on my part knowing how NiCad batteries can fail.
A long time ago when I used more NiCad batteries, I used to "zap" them with a charged cap to remove the short and be able to use the battery again.
