grammar - Difference between ば and たら in this example

明日都合がよければきてください.

東京へきたら、ぜひ連絡してください.

Why in the second example we can't replace たら with ば but in the first one we use ば.

Aren't きてください and 連絡してください both requests or is there a difference?

Answer

My impression is that ば presents a vague or hypothetical situation/state, and ら applies to actions and is similar to "when."

明日都合がよければきてください.
If tomorrow happens to be convenient, please come.

東京へきたら、ぜひ連絡してください.
When you come to Tokyo, please contact me without fail.

With those implications, the opposite doesn't make much sense.

"When tomorrow is convenient, please call." The conditional is too strong and confident for the unknown situation.

"If perhaps you come to Tokyo, please contact me without fail." The conditional is too weak, and it is modifying an action.