Is it because of the reliability and durability,
Indirectly, yes. It's partly about the duration of the warranty and your confidence that the manufacturer will still be around to replace defective panels in ten years time. Or twenty years time. There's a bit of a rant here about why the actual length is possibly not worth bothering about: decent panels won't fail; decent companies go bust anyway; you're going to forget you have the warranty (or not measure the performance); and panel prices are (currently) dropping so replacements will be cheap (and likely more efficient). But... you can't get decent panels with short warranties so it's not necessarily useful to apporach things that way.
The other part is the "does what it says" warranty. Some countries have decent consumer protection laws (Australia, for example) and oddly a lot of those cheap panels aren't sold in Australia, only in China and the USA. I've never tested the cheap panels, but the mere fact that no-one is willing to import them and sell them here is very suggestive to me.
I have talked to one person with a pair of "500W" panels that are huge but apparently he was given them free by whoever bought them direct from China because the actual output was similar to the 200W panels they replaced. So not worth the hassle of having giant panels in a space that would fit four or more actual 200W panels.
In Australia we also had a pile of panels fail due to water ingress, and at least some of that was due to cheap backing material but much was just poor construction or damage during installation (microcracks aren't necessarily easily visible, you just have to handle panels carefully).
