This answer is kinda a sum-up of other answers (namely this and this) plus my experience as a DM.
Two main statements on your question show us why your BBEGs are being targeted first:
The enemy is clearly the biggest threat
And all that will result in the boss dying after 2 rounds
So, let's change them.
Make the minions a bigger threat than the boss
"Then aren't the minions the actual bosses?" - No. Either because the boss is the one spawning these minions or because they are only immediate threats, while the Boss is a long term threat.
Here are a few ways you can make them a bigger immediate threat.
Already mentioned in other answer. Make them High Offensive CR and Low Defensive CR monsters. One easy way to do it is rogue-like creatures with either ranged attacks or daggers going for your squishy members - such as your party's Wizard. The type of these creatures depends on your party level, but it can go from weak kobolds or goblins early on to assassins later on.
My application on this were literally Rogue minions that would shoot the party. The boss would run and dodge while the Rogues weren't targeted, since the party was only targetting him, he could let the rogues do the damaging job, while focusing on not dying.
As a house-rule, you can make spellcasters that are "charging" on more powerful spells - and let it clear they are doing it to the PCs. If your party decides to ignore these spellcasters, not breaking up their concentration, they will suffer harsh consequences, like a 2-levels-higher-than-usual-Fireball. While this might not force them to kill the minions, it will force them to spend some resources, either damage or crowd control, on them.
This was done in a "the enemy is a cult" setting. The spellcasters would cast "Save or die" spells if allowed. The PCs didn't understand how dangerous it was until they actually allowed a spellcaster to finish his 2-rounds preparation and cast a spell that instantly killed one of the party members, but welp. Make sure to let it clear how dangerous it is so it does not turn into an unintentional TPK.
The minions don't need to be highly damaging to be a threat - they could be constantly casting Sleep, Dominate Person and forcing saving throws on the PCs, in such a way that the PCs just can't focus their attacks on the Boss without being constantly disrupted.
My only experience on this was a TPK, sorry. Still, this exemplifies what ignoring the minions caused, so I thought it is worth mentioning so you can try.
Make the boss more durable
This intends to solve the second statement - that the boss dies after 2 rounds. If the boss doesn't, the players will be less inclined to focus fire on him, as they will spend too many rounds handling too many threats.
The minions buff the boss AC
This one is easy - the minions are either spellcasters concentrating on a spell that increases the boss AC or constructos that generate a "Power Shield" or anything on these lines. It could be taken to values that it might be too hard or even impossible to hit without criticals, but it is not needed to. Making it just hard enough to hit so that it takes too long to drop him is enough.
I've used it on a homebrew Golem with 4 small tower-like constructions that increased its AC from 10 to 30. At 30, it was obviously way too hard to hit, but the party didn't need to kill all the towers - 2 were enough. While these towers are not exactly "minions", the concept can be expanded.
There are a few ways to make this one. The minions having spells that directly heal the boss is one. The boss having an action that drains the soul of the minions, healing himself is another. The first forces the Party to kill the minions, the second just makes it easier for them to kill the minions first - usually the minions will have less AC than the boss, so taking the HP out of the minions is easier than taking the HP out of the boss.
A necromancer with a bonus action that kills one of the minions and restores that much HP is a good example.
Create a boss that is high-defensive low-offensive CR
Pretty the opposite to the Glass Cannon minions. Make it have a huge AC and HP, but with low DPR. Then make it spawn the glass cannon minions as a Rechargeable action. I've done this with a homebrew Kobold Shaman. He would do almost no damage to the party, but would frequently summon weaker kobolds to do this job. The party would have to constantly kill the kobolds and then go back to downing the boss' HP.