The Democrat Party, unlike the Republican one, is made up of very disparate interests, that don't always line up with each other. The Republicans have stabilizing forces within their party that revolve around things like trusting the invisible hand of the market and religion that the Democrat Party haven't really successfully replicated yet.
The conversation around big media illustrates this well. When talking about the Republican side of the conversation, really only OAN, Infowars, and Fox seem to come up. When talking of the Democrat side, however, people end up talking about the 'mainstream media' (aka almost the entire field of nationally recognized news). There is a focus of message that comes from limiting yourself to relatively few sources that doesn't happen when talking about almost an entire industry. Of course, that's not to say that Republicans messaging works and Democrats doesn't because of the amount of voices contributing to that message, but it does illustrate, I feel, how disparate the Democrats message is compared to Republicans.
Some of that unfocused messaging is probably due to policy, (arguing for a by-gone era or a return to something past is certainly a more concrete picture than attempting to actively and dynamically create a vision for the future in the present), while some is perhaps due to the big tent nature of the Democrat Party (LGBTQ+, Green, 'socialism', Unions, and many more. Its a big tent that requires juggling many hats at once. Whereas the Republicans can just basically invoke God and say 'business tax cuts' and pretty much everyone is on the same page.
I don't know if I'd say that the Democrats message is inherently 'diffuse, often confused, and unfocused', but it is pretty hard to argue that it isn't if compared relatively to the Republicans.