It has to do with the new reward curve. It used to be linear. (1000 SP being worth twice as much as 500SP whenever or wherever (post or comment) the vote was cast).

The reward curve now slopes up slightly first and then turns linear later.

To get initial rewards started you need more SP than before. It's introduced to incentify better curation on posts and combat abuse, but voting on comments has indeed taken a hit because of this.

I have noticed this myself too. I had to increase my vote percentage on comments. I will, therefore, be more selective which one I choose to upvote more than before.