This can actually be a good reason to focus either on the second reading, or on the OT and Gospel.
I have heard it said that you can’t expect people to remember more than 3 points when giving a talk. Presumably this is in a classroom, or at a lecture where they are already somewhat knowledgeable and primed to learn - I think 3 may be a high estimate for the average mass attendee. If a homilist has a particularly powerful message, it may be wise to simplify the homily to expand on just that one point, rather than dilute it explaining sometimes tenuous relationships between the readings.
Having said that, I personally prefer when the homilist does discuss each reading. So many homilies have core messages I have heard before, or for other reasons don’t particularly move me. When the homily switches gears to talk about each reading, the chances are better that there will be something new to me, or that for whatever reason I’ll have an easier time latching on to.