The Wolkenritter are Hayate's children. Because she likes someone doesn't mean she thinks they're the right person to take care of her children; nor would she want to place the burden on someone whom it would weigh too heavily on.
Nanoha and Fate are currently twelve, have troublesome personality traits (Fate's difficulties with affection, Nanoha's aggressiveness), and adding five more wheels to the Nanoha/Fate relationship isn't doing them any favors. Arisa and Suzuka are not mages and removing the Wolkenritter from a mage environment and the ability to practice their greatest skills isn't just a waste, it's a hardship: fighting for the Bureau helps them feel they're atoning for previous crimes in a way life on Earth won't. Most of the other cast members by StrikerS (chronologically we haven't even reached the epilogue for A's) aren't even known to Hayate yet. Yuuno has a large and difficult and non-combat job (see comment on Arisa and Suzuka) that consumes most of his time and attention.
So we're really down to just Chrono: combat mage, hypercompetent, strong sense of duty, older, proven responsible, and with his relationship with his adopted sister Fate, proven able to take in and treat as family people from outside his life even if he once fought them. Which is really the killer qualification; nobody else has shown quite that much maturity and forgiveness yet.