Not a lot of people may think about something like this, but once it's in the brain you may tend to go, "hmph, that is an interesting question."
Does anybody notice how certain major sports leagues have minor league affiliates and others don't? Major League Baseball has the MiLB with an extensive farm system of A, AA, AAA, and so on; the NHL has the AHL, along with numerous lesser leagues around the U.S. and Canada; the NBA has their own developmental system known as the D-Leagues or NBA-D; finally, Major League Soccer even has affiliates (although not direct) below them on the soccer pyramid in the form of the United Soccer League.
What about the NFL?
The National Football League is one of the biggest sports leagues in the world, so how is it that they don't have a developmental system? Sure, I bet a lot of people considered the USFL (United States Football League) the "minor leagues" in the 80's until the USFL decided they wanted to be bigger than the NFL and then proceed to implode on itself. But in all honesty, there's no gray lines between college football and pro football. If a college player who is drafted is cut in preseason, where else can they go? To the gym so they can train and wait another year? Sadly, that's the case. Unfortunately for these guys that don't make it to the main rosters of their team, an idea of having a minor league system implemented into the NFL would never work.
Why do you think that is?
The first argument that would probably come up is the length of the season; each NFL team plays a minimum of 16 games. There wouldn't be enough of a legitimate time period that would make up a healthy season of minor league football. Also, roster sizing is a big issue, as there are the ideas of second-stringers and third-stringers in football. Most big league systems have over 80 games (with the except of MLS, which has 34) and they have the capacity to make moves and transactions. For example, Major League Baseball has 162 games, and since their rosters are smaller than football rosters and the minor league season is shorter, you have the capability of "bringing guys up" when there is injury. It's the same thing with hockey; they have 82 games and a smaller roster as well. Oopsie daisy, NFL.
Here's another thing to chew on: football is one of the more intense contact sports out there; you would have the "minor league" players going out there and getting pummeled and maybe losing their chance to remain healthy to try out again for the pro team the next year. Sometimes fresher guys that don't play for a season and then train on their own are usually the best guys to go after if adjustments are needed for the next season. With that being said, you're probably thinking right now that having a minor league would actually benefit the guys that were cut in preseason so that they're still playing. But think of this next part of my argument:
I'll be playing devil's advocate with this one--wouldn't that somewhat eliminate the concept of owning a "practice squad" since a lot of those guys would be playing for that lesser team? Another problem that would stem from this would be that more money would have to be incorporated into hiring other guys to work with each major squad that exists in the farm system or in the pro system. Basically, a lot of money would have to be involved to make a farm system happen. In this day and age where money is big, that's never going to happen to any football franchise. Period.
In looking at the Arena Football League and what they went through with an attempt to create their own developmental system (known as AF2...how creative...), you can tell why making a developmental system in football would never work. One of the big reasons was money, as well as the particular areas the teams were marketed in. They would literally put teams in awkward areas that already had a huge section that was more faithful to an NFL team as well as an area already housing an AFL team and try to expand more from there. It was too much football--and I know some people out there might say, "aww, c'mon, you can never have too much football! Heh heh heh!" Yes, I said it. Now go take another swig of your beer and go back to watching Monday Night Football.
However, it is safe to say that there were a lot of guys that somewhat "developed" in other football-related leagues such as the Arena Football League, the Canadian Football League (which I highly respect since the NFL practically rules the world and the CFL are still around despite that) and in NFL Europe when that existed. Although I wouldn't call these three leagues "minor," they moved to the bigger and better things in the strongest football league in the world. That's all that matters, right? It's just a shame that there isn't a second chance, per se, in having a minor league system to fall back on. You either make it or you get cut. That's all for now and for the future.