I think it’s because of the way annual plans are marketed in relation to VPNs. For example, NordVPN’s homepage currently is offering 74% off, but when you click through, you have to buy 2 years of service to get 74% off and no mention of the 2 year plan is made on the ad and there is no discount at all on the monthly rate, which is 3 times the cost. So you have the choice of roughly $4 per month if you lock-in 2 years or $13.99 per month vs Mullvad’s $5.20 flat rate per month without agreeing to 2 years.

It’s just been my experience that when you have to prepay 2 years up front you are giving up any recourse if the service starts to suck, the company goes bankrupt, etc. It makes sense to be a bit skeptical when the discount for 2 years is 74% vs a normal company that gives you a 10-20% discount on a bulk purchase.