Its difficult, isn't it. The First XI skipper will obvious want to have his very bet go at winning every game so may not rush to trust new players, but in doing so will never find anything out about them to know whether or not they are up to playing/capable of performing at the higher level, so he ends up trapped in a cycle of rotating the same 12-14 when available regardless of form or attitude. Which, given that the differences in standard are actually pretty minor at most clubs between levels, is a poor way of doing things.
Obviously, there is another factor that affects this, which is that first XI's tend to have a disproportionate number of people who are genuine all rounders - that is to say, people who are good enough to bat top five and bowl 10+ overs per game - because they will have first pick of the most naturally gifted sportsmen. This can mean, as happened when I came to play for my current ide (who were at the time somewhat better than we are now) that you have two opening bowlers who bat one and four (one of whom the kind of guy who would happily reel off 26 over spells most weeks), a first change seamer who hits the ball a country mile and tends to come in five, a slow left armer who opens and an overseas player who bats three, keeps wicket and bowls very good off breaks. That first season, we regularly won games with five or six guys only ever fielding...