One of the major problems of human behavior is that until an exploit is fixed, there's little reason for people not to use it. For instance, people wanted an easy way to listen to music, and downloading music off the Internet became popular because it was much simpler than buying the CD and copying it to their computers. Instead of trying to fit their models to new systems, record companies began suing people and introducing copy-protected music, standing in the way of progress rather than moving forward.
It's pretty clear that Jordan Lyles is the best prospect the Astros have. Not only has he made several MLB Top 100 lists, but he's also just 20 and has already proven himself ready for (at least) a full-season in triple A. He's outpitched every bit of competition he's had in spring training, be it Nelson Figueroa, Aneury Rodriguez, Lance Pendleton, or Ryan Rowland-Smith. If the Astros wanted their best team on the field from the start of the season, it's obvious that Lyles would have a strong argument to be their fifth starter
Unfortunately, life isn't that simple. Lyles has yet to pitch in the big leagues yet, and thus his arbitration clock has not begun to tick. Because of MLB's arcane rules, letting Lyles pitch for the big club from the start of the season would lose them almost a full year of his services. As players gain service time on the major league roster, they paddle closer and closer to arbitration. After they accrue three full-seasons, they are guaranteed the right to argue their salary in front of an arbiter.
Because of roster shenanigans, not letting a player reach the third year (or the super two line, which is a bit more complex) he needs for arbitration is fairly simple. All they have to do is call him up after the projected date in which he would fall short of qualifying for arbitration: the end of his third season in the majors. By doing this, the club keeps the player cheap for an extra season, while also delaying the march to arbitration, where players start getting paid more money. As if that wasn't enough incentive, the team also gets that extra half-season or so of time in the majors for free.
If the Astros were seen as a contender this season, one that could benefit from putting it's best team out on the field from the start, it would be a much harder call for the team to snub Lyles at the start of the season. At that point, they'd have to measure long-term finances against possible short-term ones: the extra money associated with both having a good team (merchandise and ticket sales up) and making the playoffs. But as the Astros don't really have the look of a competitive club at this point, the team has even less reason to put Lyles on the roster at the start of the season.
Now that Nelson Figueroa has all but won the fifth starter job following Lyles' demotion to minor league camp, Astros fans should be pleased. The last thing the Astros should be doing is throwing away a potential year of having Lyles in the big leagues at his peak away for a team that is projected to finish anywhere from dead last if you're a pessimist to fourth place if you're an optimist.
Until the system is fixed, the Astros would be wise to exploit it.