I don't get how this conversation turned into a big team-small team thing.
The lack of performance in the McLarens at the moment is not the biggest reason they are unable to attract large sponsors, but it is a major factor. Look at the teams that are ahead of them:
1. Mercedes: Bankrolled to the point of no expenses spared, by the manufacturer. Need for sponsorship is minimum, but nice to have.
2. Ferrari: Essentially the same as the above, except they are going through a period of complete and utter mismanagement.
3. Red Bull: Team owner is a bottomless pit of money, however, thanks to their fantastic performances over the past half decade or so (a bit more than that), are able to attract extra sponsorship, albeit through their engine supplier (in Nissan/Renault: Infiniti).
4. Williams: Now we're at the level where you have a team that is arguably smaller (but no less important) than McLaren, and have had sponsorship troubles of their own. They have a smaller operating budget than McLaren, and most likely also charge less for space on their car for two reasons: The operation is smaller, and because they haven't had the same success as they had in years gone by.
5. Force India: An even smaller operation than Williams. Their main sponsorship are various subsidiaries of the team owner, plus a few others. They, along with Sauber, and arguably Lotus, have a fairly decent technical setup that has allowed them to punch above their weight in recent seasons, however, all of them have struggled at some point or another for resources. Force India is the most stable of this group, but they still have money problems of their own.
Now you add McLaren into the mix and you are immediately able to see that they sit in between two groups of teams right now.
On one end, you have Mercedes, Ferrari and Red Bull who are all massive operations, but do not have to particularly worry about sponsorship so much, due to the natural funding they receive. However, due to name/stature, the comfort of knowing that they don't actually have money issues, and performance, they can ask a high price for a sticker on their car, and afford to hold out for it.
On the other hand, you have Williams, FI, Sauber and Lotus, who yo yo between themselves in the performance department, and sometimes have exceptional years (I'm talking about recent history only). Being smaller (and again, I reiterate, I just mean smaller in size - Especially in the case of Williams, and arguably Lotus), and not having works relationships, they will look absolutely anywhere for money, otherwise it puts the entire operation in jeopardy. Thus they can undercut the bigger teams to get sponsors on their cars because it costs sponsors less to do it.
And smack in the middle of this, you have McLaren who have a massive operation, extremely poor results over the past half decade (as a fan, I'd argue even longer ), have diverted resources into a new car company, and have a fairly complicated and political ownership environment. Couple this with the fact that they are no longer a works team (up until this year), and you can see from their point of view, why they'd put a larger price tag on the car for logo space than say, Williams. However, due to recent results it's hard for them to convince someone to pony up that cash. If they begin to accept a lower amount of money for title sponsorship, then it wouldn't be an easy task to increase the amount they get if their (projected) value increases through better performances and TV airtime. On top of that, it could be a case of sponsors wanting to do multi-year deals at low rates, therefore, making the choice difficult, if indeed the team's performances do imminently improve.
There are a lot of other factors, I am sure, but on the face of it, it's fairly straightforward why it's difficult for them to get what they want, because they are demanding A, the sponsors are willing to pay B, and the value for money the sponsor will get is sitting somewhere around D or E.
I think if the team begins to show promise through this season then they will have an easier time finding someone who wants to slap their big logo on the car, and it feels like this is what big Ron is waiting for.