My guess would be that back in the '80s when derate info was not often available and the contaminated runway data was mostly generic (derived from NASA's testing of a Caravelle in the '60s) it was regarded as pushing things just a bit too far to take off with less than full thrust on a contaminated runway. Enter EASA, and contaminated runway take-offs became not 'Captain's discretion' but effectively a normal take-off, and supported by better performance data. This combined with widely available derate performance data changed 'pushing things a bit too far' into 'really its just a normal take-off' for derates, which were quite a new thing, but didn't quite make the bar for the old-style flex take-offs, maybe there were some residual nerves that a flex take-off on a contaminated runway still wasn't a brilliant idea. In reality, I can't see why the logic applied to derates should not also apply to flex take-offs, if you agree with it. In fact, I would say the reverse applies. Taking off deliberately with less than full thrust on a contaminated runway still doesn't sound like a good idea to me.