Attention fades quickly
Back in my lawyering days I ran a designer paper goods company at night and on the weekends. It was a cute product - hip versions of the paper bags we'd made for textbooks as kids - but a terrible business. Possibly because it was a terrible business I learned a ton and discovered many things through the mistakes I made.
One of the things I learned was that attention has a very short half-life. I was convinced that every (occasional) bit of media coverage would be the thing that would help the company and product break out and every time there'd be a bump of some sort quickly followed by a reversion to wherever I was previously. I never paid close enough attention to quantify it but I'd say that the half-life of the bump was about one day meaning that it would decay to almost nothing within just a few days.
Maintaining consumers' attention takes effort
Now that I work at a consumer-facing company with a significant marketing budget I see how naive I was about how the world works. We spend lots of time and energy raising awareness of our brand and product and understand that it's an accretive effort but a never-ending battle. It seems to me that the same holds true for developing business relationships: It takes constant and never-ending effort to make sure your company stays on the radar of prospective partners so that they think of you when an opportunity arises.
Maintaining your networks' attention also takes effort
The same goes for personal career development. These days I get emails semi-regularly from friends and acquaintances who are looking for new career opportunities. It's my absolute pleasure to help whenever I can but I've noticed that there's a half-life to the thoughtfulness I give to these requests. I always respond and usually have one or two useful things to pass along over the first week or so but, if I'm honest, I inadvertently let their job search slip from my mind.
Unfortunately the techniques used by companies to stay "top of mind" (e.g., frequent marketing across lots of different channels) aren't available to individuals. As I think about it, though, there are lots of useful and natural ways to keep up contact once you've made your original ask. I've talked about it before but saying "thanks, here's what happened" is one. Giving an update a month or so later is another. Stepping up your efforts to find useful and relevant articles to pass along and to find helpful introductions to make is a third.