It's the stigma. People see BLM as the easiest way to do things and therefor instantly try to cater every fight to BLM's because that's what they're used to. I'm assuming you have seen the video of a JP group beating garuda with all jobs present, right? This proves that it's more than possible, but not many are willing to 'think outside the box' like that just yet.
It's the same reason most people use the SAME exact strategies to beat endgame content. Once one person does it, and it is a tried+true method, everyone else chooses to do it that way too. (Examples: The BLM burn Garuda, or the BG setup since they were the first to beat it, or the wall-stacking method to beat Chimera.) Everyone sees how it's possible and starts trying it that way. After the first group beats something and reveals their strategy, it spreads like wildfire until it is the 'known' and 'accepted' strategy that most groups attempt to adapt to.
Another reason is because gearing a class takes a lot of time and money. Double (or even triple) melding an item takes a lot more gil and/or time than many who play this game are willing to commit to. Therefore, they have to really choose... 'which class am I going to try to gear the best'? Many choose BLM, and for them, their melee gear will never compare. There are very few of us that can take the time to double/triple meld multiple classes. So, when it comes down to... "Hey, let's try a different strategy," it may inevitably look bad because the 'new strategy' involving classes that aren't geared as well simply won't put out the same kind of performance.
The stigma of BLM being the 'best' for everything also puts all other classes 'out of practice'. If you don't play other classes very often, you just might not be as efficient at them... and that is another reason the other classes may appear inferior when people actually try to play them. I encourage thinking outside the box, especially if you have a class you'd like to play that others don't often look for. When SE designs these fights, they make sure there is a way for melee classes to survive and thrive. You just have to find it... and the unfortunate truth to this is that the BLM/range niche is often easier to discover than the 'up close and person' one.