"It just rained too hard for too long a period of time," Sveum said.
The game was called after a 48-minute delay.
"It was terrible -- I thought I was on ice skates," Sappelt said of the field conditions. "Every time I tried to throw the ball, I thought I was going to slide."
Both the Cubs and Rockies began the day with 94 losses, and the Cubs' magic number now is four. They need four wins in the remaining eight games to avoid a 100-loss season, which would be the team's first since the 1966 Cubs lost 103 games.
Sappelt, hoping to earn a spot in 2013 as an extra outfielder, got things started with his home run in the first off Jorge De La Rosa. Two outs later, Alfonso Soriano walked, moved up on a wild pitch and scored on a throwing error by Rutledge, who fielded Castro's grounder but threw well over first base. Welington Castillo hit an RBI double to make it 3-0.
Rosario homered off Cubs starter Chris Rusin (1-3) in the second, but Castro made it 4-1 with his homer with two outs in the third. The Rockies answered again as Colvin hit a two-run triple.
Darwin Barney hit an RBI single in the Chicago fourth to go ahead, 5-3, but the Rockies took a 6-5 lead in their half as DJ LeMahieu hit a RBI double off Rusin, and Rutledge hit a go-ahead two-run double off Rafael Dolis.
The Cubs now are 0-14 on the road against National League West teams.
"It's always tough to win in the West," Sveum said. "The bottom line is we haven't been able to get anybody out, either. ... Tonight, to score that many runs early and then give just give it right back, you're not going to win many road games when you're giving up that many hard-hit balls or balls hit out of the ballpark."
The Rockies pounded 15 hits -- six for extra bases -- in the abbreviated game.
"We came out swinging the bats really well," Sveum said. "We couldn't get them to make any soft contact, that's for sure. There were some balls hit pretty hard on their side. Even the outs we got, we're lucky they were at people."
In the first inning, Rusin got ahead of hitters but he wasn't able to do that consistently. The lefty didn't blame the rain.
"You just put [the weather] in the back of your mind, and pretend it's you, the catcher and the hitter," Rusin said. "That's what I try to do, and I didn't do too well."
He has one more start this season, closing the series against the D-backs on Sunday in the Cubs' final road game.
"It's been a learning experience, a good experience to get your feet wet," said Rusin, who spent most of this season at Triple-A Iowa. "At the same time, you try to do well. You just take whatever you can get from it and learn from it, and figure out what you have to work on next year.
"[I've got] one more -- try to end solid and end on a good note," he said.
Sappelt is one of several players to be considered for next season. He could be a right-handed bat off the bench.
"In my mind, that was the role I could take over if I could get my stuff together and perform well," Sappelt said. "Whatever happens, happens."
Sveum said he'll meet with Theo Epstein, Cubs president of baseball operations, and general manager Jed Hoyer the day after the season ends to discuss the roster.
"You have a really good idea," Sveum said when asked if he'd seen enough of the roster to evaluate the players. "You've definitely got an idea of the character of people and the pitch ability, making pitches, being able to repeat things. Does this guy need to develop? Does he still have a chance to make the team? I think we've seen enough of everybody for that."