1) More time = more competition. The more time has passed, the more developers have been able to learn to code.
2) More competition = better marketing by the competition. If your marketing skills stay the same, it's likely that you'll see lower/diminished call for services if your competition's marketing skills increase.
3) More marketing = more generalization. If everyone is advertising the same services, what are the differentiators? Price and design are easy ones for even the most uneducated clients to see. Architecture and "quality" are not.
4) More generalization = more commoditization. Everyone does "it" so you have to do "it". And you'd better not charge too much more/less than everyone else.
5) More commoditization = lower profit margins.
6) Lower profit margins = adding new services to make up those costs.
Sorry for the ramble - just thinking out loud a bit. I'm probably off on one or more of these but I like the thought process.
good observation. On the other hand, one would expect the demand to be rising, too - everybody and their dog wants a mobile app.
The reason I posted, though, was to see of other mobile devs see the same trend.
I'm not looking right now but I check craigslist from time to time and there are more iOS jobs now than 12-18 months ago when I was looking.