There are several reasons behind FanPro's current approach to sourcebooks, the biggest being fan-demand and economics. For as long as there's been Shadowrun people have demanded more knowledge of the Sixth World. This is multi-pronged approach: location books and storybooks and sourcebooks that mingle the two. Though a lot of the original location books weren't the best of sellers, from what I've gathered this began to change in the latter days of FASA and through the current FanPro run. Most fans seem to have appreciated both the latest background books (YotC, Threats2, etc) and the locations books (SoNA, T:WL, etc) which means this is a trend to be followed. Furthermore these books help integrate and develop one of the reasons that SR has the fan following it does: the metaplot (or to be perfectly correct "metaplots").

The other important aspect is economics. Background sourcebooks sell far better than adventures. Adventures by their very nature sell less than sourcebooks, being aimed at one person in the average group rather than the group in general. This is pretty much standard in the industry and something that has become a major trend in recent years (look at the ratio at White Wolf or SJG), it is especially limiting for smaller publishers who can't afford to have a weaker selling title when it could be using that production slot for a title with a better sales potential. That being said adventures do serve a function in SR as they allow players to contribute in their own way to metaplot events and to get involved in stuff that affects the gameworld at large and FanPro will probably continue to do them at the current rate.

Note FanPro has basically 3 formats for adventures: the big interlinked adventure campaign books (Harlequin, Brainscan, SotF); the three adventure packs (Wake of the Comet, Predator and Prey, First Run); the track adventure style campaigns (Mob War, Blood in the Boardroom).

A lot depends on the city but an adventure book is not the ideal place to introduce a city (namely because of wordcount limitations).

Generally speaking, especially if you're a newb, a section of a book is a better place to start submission-wise. An entire book is a huge responsability and one that only a few freelancers are allowed. You're better off first trying your teeth out on something that showcases your ability to write to spec and within wordcount, follow guidelines and shows you know SR, that you can integrate other people's input and be flexible regarding your own ideas, and finally shows off your writing skills.

Didn't Tripoli get nuked?

This is also the biggest reason why submitting adventures is suggested.

1. They have an established format; if you can't follow the format, it's generally likely you'd have a hard time with something that doesn't have (but will need) any sort of format.
2. That you can come up with a plausible adventure demonstrates your knowledge of the Shadowrun universe.
3. Adventures also average 18,000 words; if you can write that much, then you can write smaller pieces, particularly sections of books.
4. Adventures can also, quite frankly, be a pain in the ass. If you can make it through writing that, remembering to include possibilities and dice rolls for almost every possible contingency you can think of, then you can probably survive writing something with a bit less detail.
5. They're also a chance to see if you can take direction. While you're definitely given some latitude in how you'd like to see things go, if the line developer wants to see something different, you need to be able to deliver without kicking up a huge fuss. Certainly if you feel something would work better your way, you could enumerate your reasons why. At the end of the day, though, it's the line developer's call and giving him hassle about it will guarantee that your future with FanPro will be a short one.
6. Because adventures aren't expected to do too well, there's more room to try out unknowns and any possible problems that might crop up due to that (good proposal but crappy execution of the final product, grossly missed deadlines for whatever reason, a writing style that gives an editor fits, etc).