A city has a maximum number of residents or jobs that it can support for each type of zone (broken down by wealth level). For example, a new city can support 20,000 R§ sims, 2,000 R§§, 1,000 R§§§, 3,000 CO§§ jobs, and so on. These are referred to as "demand caps". Once the demand cap is reached for a given category, no more buildings of that type will be built. Certain buildings provide demand cap relief, increasing the demand cap by a certain amount for one or more categories (e.g. a City Zoo provides 8,000 R§ and 16,000 R§§ demand cap relief, increasing the maximum population your city can support).
There are only a few buildings that provide Industrial demand cap relief, and none that provide IA (agricultural) relief. However, Industrial relief is provided by freight connections. Several times a month, industrial buildings sent out freight shipments, and each one that successfully reaches either a neighbor connection or a seaport provides 20 demand cap relief for each industrial type.
Agricultural demand cap is a special case. For every other demand type, the demand cap is simply compared to sims or jobs of that type. The agricultural demand cap, however, is reduced by all other jobs. The starting agricultural demand cap is 30,000, so if you have a city with, say, 2,000 CO jobs and 3,000 ID jobs, that would only allow 25,000 IA jobs, and as the rest of your city grows the limit would keep decreasing. Once the total number of jobs in your city (both agricultural and otherwise) reaches the IA demand cap, no new farms will be built. There's no way in-game to see what the current demand caps are, but the starting value should give you an idea.
(Source of this information is the Prima strategy guide; I can't guarantee that these numbers are still accurate, but they should be close.)