Because the first word is "multimodal" :^) and also because MLLM is the established initialism.

Ok our options

  Multimodal large language model
  Large multimodal language model
  Large language multimodal model
  Large language model (multimodal)

I prefer 1, because this is a multimodal type of an existing technique already referred to as LLM. If I was king, I’d do Omnimodal Linguistic Minds, but no one asks me such things, thank god

The bikeshed color argument never ceases to be relevant. Would you say "large language model multimodal"? I doubt it.

what's a language multimodal model