Essentially both ratchets are used to ensure that the encryption keys being used change for every single message. This ensures that if a message encryption key is ever compromised, an adversary can't also go back and decrypt any previous messages (forward secrecy).
If you just did the symmetric-key ratchet for X messages in a row, you'd still get a new key for every message, but the keys are predictable if an adversary obtained one of your chain keys. If an adversary did obtain one of your chain keys, then they could decrypt every message sent from that point onwards, until you used a DH ratchet to make a new chain key that the adversary could not predict.
This is fine, and is what happens when you send multiple messages in a row while the other participant is offline anyway, because you cannot perform a DH ratchet step without receiving a public key from them. It just means that you risk the adversary decrypting a few more messages (if they ever compromised your chain key) than they would be able to if you did a DH ratchet more regularly. The more often you can do a DH ratchet, the more unpredictable your encryption keys are, making it harder to compromise more than a few at a time.
