It costs money to create the app (if you're talking a universal app that works on ALL smartphones, just think of how much it would take just to create something compatible with everything out there). It costs more money to maintain the app, make bug fixes, etc. And, of course, it takes money to promote the app and ensure people actually use it. That's all money that Yahoo probably doesn't need to spend. They already made an Android app, but people were still using the mobile browser to access Answers, so they discontinued it.
It is not probable that a 'universal' app will be made, considering virtually everyone accesses YA from their phone browsers.
If you're having trouble with mobile YA, scroll down to the bottom and click the Desktop link. This sets up the site to look as it would on a computer, making things much easier and giving you access to features you normally wouldn't.