It's simple in my opinion because 100% population in the world are directly connected with finances, but 100% peoples aren't directly connected with other sectors. We can't imagine our life without money, so when a blockchain application built financially based then they become more popular easily. But other sectors like education if built blockchain application then it will be limited to the teachers & students.
More importantly, blockchain becomes most popular by bitcoin which is financially related. That's how financial blockchain applications become popular.
I think financial application is becoming more popular because some people are not directly connected with finances, but they would like to be. Legacy financial system has showed us that in order for the centralized system to be effective, it needs to cleared of undesirable participants, who, for example, don't have identities. Those people can't access the system simply because they cannot ask for permission. Bitcoin, however, is open for everyone, it is completely permissionless system.
This is the first and most important use case of blockchain technology, it gives people what they really want - the right to express opinions and make transactions without having to ask anyone.