In Roto, one team can take off early and never be caught. In H2H, you have another chance to catchup playoff time. The early lead means nothing. Gotta play all year.
This is a very overused argument against roto in my mind. In my main league last year, the eventual winner was in 5th place, something like 25 points behind the leader at the All-Star break. He didn't overtake the leader until the last week of the season, which went back and forth every day until the season ended.
There are also plenty of rule tweaks that can be used to keep teams interested all year in a roto league. Two I've seen:
1) the team that gains the most points from the All-Star break to the season end gets some of the money payout.
2) the overall pool of money is funded based on your finish in the league. In other words, you pay more if you finish last than if you finish 3rd to last.