pi4t: ...Or that they were talking about BG2, not BG1. The question is about both, and given that there are a lot more options available in BG2 it would be surprising if there weren't a larger number of people whose favourite class choice only exists in BG2. Or maybe they just care more about what class they choose for BG2 as they prefer it to BG1?
Hickory: It's hard to imagine so many people would simply ignore the actual question. But you may be right.
The question was "What is your favorite class for the primary character for both BG1 and BG2. " While that could (and technically should, I think) be interpreted as "what's your favourite class for the main character out of those which are available in the same form in both BG1 and BG2", it seems to me that the OP was meaning the rather less...arbitrary question "what's your favourite class for BG1, and what's your favourite class for BG2?" With that interpretation, people were just answering the second part of the question - and, admittedly, ignoring the first part. My theory as to why is that there are a lot more class options in BG2 with kits and so forth, so it's much more likely that one of the options will particularly appeal to a player and they'll offer an opinion here. Personally, I don't have a very strong opinion on which class I most enjoy in BG1
or BG2, so I didn't contribute to the conversation. Perhaps in the same way, people who only feel strongly about an option which is available in BG2 didn't feel the need to make a comment about BG1's class selection?
Another reason that class selection in BG2 is more significant is that it has a larger impact on how the party plays. In BG1, there are less different classes, and dual classing isn't common. Thus to an extent it's not that important what the PC's class is, as the player can choose NPCs with other classes they find enjoyable. The decision of whether your favourite class is a mage or a thief isn't so significant when all your parties can contain both of them. There are exceptions (fighter>mage dual class, for instance) but they're generally either bad anyway (certain multiclasses) or only fairly minor changes from an option that is available (fighter>mage compared to pure mage).
In BG2, there are many more choices, and a few less NPCs. Thus, there are a lot of options which are only available to the PC, and the decision of which class you want to play becomes something people put much more thought into.
On another note, personally my favourite class is almost invariably "whatever is as different as possible to the last couple of characters I played in RPGs, while still being enjoyable".