Why can't I download some of the apps on my iPhone?

Okay, so I wanna download apps onto my iPhone 3G. Some work fine, but when I try to install others, it comes up with a message telling me that the app is incompatible with my iPhone? Why? And is there anything I need to do to change the compatibility of my iPhone with certain apps or something?

Basically, you need to either upgrade to a new iOS.
or
get a better iphone, e.g. Iphone 4

  • Why do some iPhones cost $200 and some cost $900? Is there like data forever or something on these jacked up price or are you literally buying a $900 iPhone when you can only use it for Wi-Fi? And why does it cost so much less when you get it with a service plan like AT&T?
  • Why do apps download on other devices when I download on my iphone? Okay so for the past few months I've downloaded some apps and I noticed that those same apps download on other devices such as my parents iPhones and iPads. How can I stop that from happening?
  • Why can't I use mobile data on my IPhone 5 with some apps? I know it's not T-Mobile. I've already tried restarting it. I've already reset all network settings. And still nothing. With Wi-Fi everything works normal. But with 4G everything works EXCEPT the mail app, safari, and sometimes google chrome. Why is this and how do I fix it? I've also have gone to settings> cellular and looked under the "use cellular data for" section and all that shows up is face time and wallet. I'm lost
  • What are some really good apps to download for your ipod/iphone? I'm getting another ipod for Christmas and just want a few ideas of some apps that are good. Mostly gaming apps