Because there are no similarities, other than a lack of evidence. Every other factor is different: environment, location, date, developments in the case etc.
Nonsense.
Both children disappeared in a foreign country.
Both children were unsupervised when they disappeared.
Both families have claimed their children were abducted.
In both cases we only have the families' word for what happened.
Both families are still looking for their missing children.
And IMO both children may well have been targeted.
The real reason why some people don't want Ben's case mentioned IMO is because they want to demonise one family while sympathising with the other. And because of the similarities that can't be done without appearing more than a tad hypocritical IMO.
IMO The one really big difference is that fortunately for the Needhams the internet was not used much 22 years ago. If it had then Ben's family would have received exactly the same vile treatment as the McCanns have IMO. The conspiracy theorists would have had a field day.