The reason iPhone can't work on Cricket is because the iPhone uses something called GSM technology, and Cricket requires the use of CDMA technology. These two different protocols determine how the cell phone connects to the provider and they are incompatible with each other. You absolutely cannot connect a GSM phone to a CDMA network because they don't understand each other.
The biggest difference that you'll see between GSM and CDMA is that GSM phones require the use of SIM cards. CDMA phones cannot use SIM cards.
If it helps, you can think of it like this, your cellular provider (Cricket) speaks the language of CDMA, but an iPhone only speaks GSM. Since they're different languages - they can't talk to each other.
Some CDMA Cell Phone Providers:
Sprint
Verizon
Cricket
Alltel
Pocket
Frawg
Leap
ACS
GSM Cellular Providers:
AT&T
T-Mobile
Cellular One
Epic
Fuzion
Jolt
Here's a bigger list:
List of United States wireless communications service providers - Wikipedia, the free encyclopedia
There are a very few set of phones that can use both CDMA and GSM protocols, the HTC Touch Pro 2 is one of them. These phones are sometimes marketed by CDMA providers (like Sprint) as "international" phones.
There are hundreds of rumors on the internet about a new iPhone coming out for Verizon wireless, which means it'll be CDMA compatible. You will need to get that "verizon iPhone" and unlock it to put it on cricket.
You could also get Sprint and an "international" phone and then put the sim card from that into the iPhone.
