There isn't an in-universe explanation for this, as in universe, this isn't the case. From River's perspective, her final meeting with The Doctor was the adventure which ended in her death. But once she was imprisoned in Stormcage, The Doctor visited her often. Some of these visits are in the Doctor Who blu-ray extras. These visits would take place in between the episodes of Doctor Who that we see, and these visits would therefore prevent them from consistently meeting earlier and earlier in River's life.
The major adventures they have, we the audience see in episodes of Doctor Who, but these minor side adventures are merely referenced on screen. For the major adventures, we keep meeting River earlier in her timeline. However, those side adventures occur in The Doctor's timeline after the events of The Wedding of River Song. As such, in universe, River and the Doctor don't keep meeting earlier in her timeline.