When you share you bet and it gets copied by other gamblers and it up been wrong, you'll be blamed and some gamblers don't want to be blamed that's why they don't like sharing their bets and not because of any superstition but they're using that to cover up the main reason.
Some gamblers are always too quick to put the blame on others and not ready to take the responsibility of their losses and when you share your bet just give them the opportunity they needed to have someone to blame when they lose, it's always better to avoid this situations.
They can blame you all they want, but it's their own fault and no one else's that they lost heir bet. You can share about your working out and someone can follow you and break their leg in the gym. So, who's to blame? Or should we stop sharing our experiences only because some unfortunate people might hurt themselves following us? I don't think so.
Gamblers are on the right mind to do what they want and it is their responsibility for their bets, at least they should make their own research to convince themselves. There are gamblers who just blame people because it is their coping mechanism and it's bad.