The short answer is that Paul's teaching in 1 Timothy 2.12 is related to a specific church (the church in Ephesus) and a specific problem. In 1 Corinthians 11.2-10 we read that Paul allows women to speak in the church there in Corinth. Whatever Paul is talking about in 1 Timothy (and there are some reasonable guesses), it's not a universal rule or teaching. What happens in Ephesus stays in Ephesus. :)

Why don't Churches/Christians of today follow the rule that women shouldn't speak in Church?

It seems like a lot of Christianity is about picking what you like and this feels like one of those moments to me.