When was the last time you asked a new client why they had decided to instruct you, rather than their previous firm, or another law firm?
And if you didn't ask them, why not?
There are plenty of reasons why clients switch firms: changes in personnel; perceptions (fair or otherwise) about experience, expertise, or value; service failure (a.k.a. cocking it up); occasionally even cost. And just like there are, or may be, any number of reasons for a client to switch, there are, or may be, any number of law firms that could have done the work / wanted to do the work / are disappointed they aren't doing the work. But they aren't.
So why are you? Weren't you curious?
You should be, because understanding why your client switched to your firm is the starting point for keeping that client.
So ask the question.