I don't think they preferred Grimm to Rondon. They just thought Grimm was cheap enough that it was worth tendering a contract to him to see if he could suddenly show something in Spring Training.
Rondon stayed healthy this year, that's the key for him. He'd been battling injuries for a season and a half, and had almost retired earlier in his career (just before the Cubs picked him up) because of injury problems. I'm sure they concluded his health was just too much of a question to pay him $6 million this year.