The quality of the mattress might be one issue. Maybe they have a better quality mattress.
Next issue is the support under the mattress. If the box springs are bad, then putting a new mattress on bad box springs is just going to make the next mattress wear out faster too. (it is better to use a kind of bed frame that doesn't need a box spring under the mattress - it usually helps the mattress last longer.) Broken boards on the box springs causes areas on the mattress to not have support.
It is also a good idea to rotate or flip the mattress every few months to avoid wearing out one certain area. People tend to sleep in the same spot on the bed which can cause dips at the point they sleep on. Rotating the mattress (head becomes foot and foot become head) can help reduce some issues. If it isn't a pillow top mattress, then flipping it completely over can help even more. (I won't even consider buying a mattress if both sides aren't usable as the "top".)
You should get a memory foam for your next mattress. It has no springs in the mattress and is less likely to end up with sink spots in the mattress. I am going to share a link to the kind my daughter has. She has had it for several years and it is just as good now as it was when brand new.
I recommend getting at least 10 inches thick. (they do come in 8 inch thick versions on other links - this link is a 10 inch thick mattress)
Deal link: Amazon
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