Why does the post office take so long to deliver a letter?

I've been trading online with cards and i sent out some stuff to prichard alabama and it got there in a week. Its been almost 5 weeks and i have yet to receive it. This has also happen from states like texas and kentucky even though i received stuff from california in only 3 days and i live closer to alabama then california (i live in knoxville) does anyone know why this takes so long?