Why do some family members dislike the simple fact that I'm opposed to selling this family vacation home?

When a property begins to cost more to maintain than it's worth in terms of actually using it the advantageous financial decision is usually to sell it. I bet if you offered to pay the market rate price for it the family would gladly sell it to you. Memories and sentimentality are luxuries in some scenarios. 

if you want it so bad maybe you should buy it

Why don't you buy it from them? Get a formal valuation so that other family members know they aren't being ripped off. It will save them real estate commission.

If you can't afford to buy it or don't want to do so then you should just keep quiet.

Whoever owns the house and holds the deed makes the decision whether to sell or not. So if the deed is in your grandparents' names and your other relatives don't have Power of Attorney to sell the property, the sale needs your grandparents' signature.  If they don't want to sell, they can refuse to sign.  

On the other side of the coin, if someone besides your grandparents have legal right to sell the place, then you can opt to buy it. 

Unless you are contributing to the upkeep and taxes on this place, it's really none of your business. You could, of course, express an opinion, but the way you phrased it made it seem like you thought you had a right to stop them from selling the place. You don't. You could say, "Oh, I hate the thought of the place being sold- I have so many happy memories around it." Or if you think the younger generations should keep the place, get together with them and make your grandparents a financial offer. Your parents, uncles, aunts and grandparents have no obligation to keep paying for this place just because of your memories. 

It comes down to use, do they use it enough to justify the cost of keeping it?  Things pass, people, places, traditions. Everything moves forward and things get left behind.  The solution to this issue is that you buy it, then it is yours to do with whatever.  I too wish my dad never sold his vacation home, nor my mothers, nor the second one my dad owned when he passed. Wonderful places that should have been kept in the family.  But the cost, the use, the crap already existing within the family.  Meant selling them off was the best way, sure. Any of us kids could have bought them, but no one did.

You have a right to voice your opinion, but so does everyone else. You seem to believe that because you are opposed, no one else should take actions to which you object. This is a family decision, a group decision, based on many factors other than your emotional attachment. Meeting with a realtor is a way of getting information that informs a decision. It's not the same as a decision made without you or behind your back. If you got the outcome you were looking for, a family decision not to sell, great. But for the future, know that expressing your opinion and your reasons for it is a right you have to afford to others as well.

Best of luck.