DD1 is 4 1/2 and nowhere near talking. We're only just starting to see glimmers of hope she may one day have a word or two.
In our case the main problem has been that the precursors to language are all missing, due to all the social processing being knocked out. So there has been no shared attention, eye pointing, social games (like peek a boo) clapping, laughing, copying etc. etc. No pointing either. There has been some mention of ASD but only in a "of course you'll never get a diagnosis because she's got too many other problems" sort of a way. Actually, I think it's qualitatively different to ASD though a lot of the issues are the same. At your DD's age, if she wanted something she had no idea that she could "ask" for it in any way - had no concept of what people were for. If she was hungry, she cried, but not "at" anyone and when food arrived she was always surprised.
She has gradually "learned" some social stuff over the last two years and we've trained her to use one sign ("food", unsurprisingly - her only reliable motivator) though not to point reliably - she will point at her drink when at the table, but isn't able to generalise that to other objects she wants in other contexts. She's now a very friendly and cuddly little person and is starting to show signs of intent to communicate.
I know another child with in some ways similar problems to her (brain damage, seizures, high levels of epilepsy meds, withdrawn, social impairment) who has picked up picture exchange very nicely at about your DD's age. He doesn't do pointing etc. and on the face of it you'd say he was less communicative than DD1 now is. But he'll move onto PECs fairly smoothly now while she's still got a one-word vocabulary. Even in kids who appear similar it really is very much down to the individual - I think as much as anything it's about how they rewire and adapt to to get around whatever the original problem is, which is an entirely organic and unpredictable process.