There is nothing wrong with using whatever data for matching as long as it is specified in the terms the user agrees to by signing up for Facebook Dating. Privacy laws prevent you from using data for other purposes or selling it to third parties.

Most likely they tried this and realised the results are biased. Imagine what data Facebook has about you, they know you live in X1 city, have visited X2 & X3 places recently, have attended X4 & X5 events, have interacted with X6 & X7 pages. Then if they try and match you with other people who have similar profiles, we get headlines such as "Facebook Dating only matches <insert minority> people with other <insert minority> people" as it's biases on hyper-local location.

Using other data such as "Person A likes band Y, we will only match them with other people who like that band too!" doesn't really make sense, as that isn't a requirement to have a good relationship... Unless by liking a band page, you really mean "I religiously go to every concert and could not date a person who does not also do that". So they might as well just do the same as Tinder / Bumble / etc and show anyone within X miles of your location.

The only advantage they really have is your friends circle, so they could use that to suggest friends of friends (of friends?), but I imagine that would quickly be exhausted if they only displayed matches from that.