First, construction in general is a pretty up and down industry. Construction booms don't last forever, and in general you need to 'follow the work'. This is great if you are young, single, and willing to chase the high paying jobs. Not so great if you have a strong desire to stay in one spot.
Second, trade jobs are usually pretty tough on the body. After a couple decades of work your body begins to break down, assuming there are no workplace injuries to put you out of commission sooner.
Third, workplace conditions are on average undesirable. Outdoors in the heat/cold, or indoors in tight spaces or high places. It's tough sell over companies that glorify bean bag chairs, nap pods, and free beer on hand.
Fourth, there's a social stigma around trade skill jobs. Blue collar jobs have gained a reputation of being jobs held by slackers or the less fortunate. Unfortunately, wearing workboots carries a stigma that wearing a shirt and tie/'startup' attire doesn't.
Fifth, and this one may seem like a silly reason to most, the drug testing (for marijuana specifically) is a huge barrier of entry. From a safety perspective, it makes sense. But for better or worse, marijuana use is becoming more socially accepted while work places (and in particular trade jobs) are lagging behind the times. Removing a marijuana screen from standard workplace testing would help revitalize the employee pool.
Sixth, and this is arguably the biggest one, four year degrees are seen as a status symbol and are being sold to use as such. They are Big Education, and are pulling on all the right strings to keep the tuition/student debt machine running. Advocating for trade jobs is in direct competition. They have enough players in their pockets (donors, alumnus, politicians) to keep the money flowing in the right direction. Selling someone on trade school/work is hard when it is seen as the less optimal path towards 'life success'.
All of these observations are based off my fathers experience as union based pipe fitter/welder for >30 years.