I think they are essentially against the idea that the block limit of 1 MB should remain. Segwit as such they don't care too much about. I think the question is rather in the other direction: why are segwit supporters (Core essentially) AGAINST block size increase ? The two main arguments they advance for this:
- only soft forks because hard forks could lead to a chain split
- there should be many full nodes to keep bitcoin decentralized, and bigger blocks will kill full nodes
are bogus.
They are bogus because a well-done HF is much less of a danger to split the chain, than an aggressive, non-consensual soft fork
SegWit as a BIP141 soft fork wasn't, and isn't, bogus, as it wasn't thought to be contentious. It's currently still the safest way to go, as most nodes are already running the software to support its activation. Another issue with hard forks is backwards compatibility. SegWit maintains backward compatibility, which is a good, conservative way of doing things.