There seem to be a lot of people lvling cooking, for several fairly obvious reasons, (though, admiteddly, I didn't know any of them when I started playing).
First, you can cook foods that boost primary stats for whatever job you happen to be lvling. You will always have a use for cooking, whether you are a mage or a melee. And you can make the food you want for cheaper than you could buy it, in most cases.
Second, you have a wider market than many other crafts. Everyone uses food of one kind or another, therefore you have a wider amount of people that will be looking for things you can make.
Third, cooking is fun and can be mixed with other hobbies like fishing. You can make your own bait from what you catch, or sell it for more than you'd make selling the fish alone.
Other crafts are immediately useful to many people as well. For instance, woodworking. You can create furniture for your own Mog House or to sell to others. You can make fishing rods, and wooden weapons like clubs, bows and arrows.
Some of the other crafts like Blacksmithing and Leathercraft are less popular because they have a much smaller target market. Therefor, they have a smaller profit potential. Also, the items made in these crafts are not usually consumables. Armor and weapons are a one-time-buy, until you need an upgrade. Food and arrows are items that people constantly need to replenish. You can have the same piece of armor bought and sold on the AH multiple times, when people no longer need it, but consumables are used up and new ones are needed all the time.
Don't get me wrong, non-consumables are always needed too, as we are constantly getting new players, and the market must grow in order to supply them with the equipment they desire. However, many of the items people want can be bought from NPC vendors, (who's prices are based on fame, not the local economy) except for HQ versions. With inflation like it is, it is often cheaper to buy from NPC vendors. If it's not, then usually the synth is not worth attempting because it likely turns a loss unless you are harvesting your own materials.
As a tip to crafters of all guilds: Try harvesting your own crystals if you have the time. Often, crystals are the most expensive part of the synthesis process. If you have signet cast on you, and start farming them yourself, you will start making a profit, or at least breaking even, on synths that used to turn a loss.
In closing, crafting is not all about making money. If you are looking to do something different, be prepared to invest a lot of gil without much return. But if it's what you like to do, then that's reward enough, even if you don't get rich off of it. Also, every person that begins a popular craft makes it harder on everyone else in that craft. The more competition you have in the market, the harder it is to sell your product. The less competition, generally the easier to sell your product, though you still might not make as much gil, since that usually indicates there isn't a large market for the items you craft. However, there are always a few synths in every craft that are in high demand. Especially from the less popular crafts, since less people are making the high demand items. If you're willing to invest your time and gil to get to the point where you can capitalize on these high demand items, then by all means, do so.