Why trial version are dying?

As I have new toy (MacBook), I need new software for it. As I’m a little paranoid about security (yeah, I know, there are no viruses on Mac) I’ve wanted to add some antivirus to my new system. I’ve worked with many such programs in the past, so I know that sometimes (or mostly) it happens that they’re slowing down computer. I’m new to Mac OS and very happy already about it performance, so I don’t want to ruin it. Obvious thing for me was to try some trial version, before I would buy it. As I’m using Norton on my desktop PC I’ve wanted to buy Symantec product also for Mac, I’ve didn’t noticed any trial version on their website, but didn’t gave up and called support. They’ve told me that indeed there is no trial, but I can buy it and return it if I’m not satisfied in 60 days. OK, it’s not bad I thought and was planning to buy it as soon as I would get back from work. When at home I’ve recalled that Kaspersky was about to release antivirus for Mac, so I’ve checked their site and yeah, there was trial. I’m using it right now and didn’t noticed any slowness (well, system is starting up and shutting down a little longer), so I would probably buy it after 30 days trial. Even though it’s 10$ more expensive than Norton.

Why companies are killing trial version? It’s so good method to sell the product.