The reasons trench warfare work in Ukraine are the Russian ISR sucks big time and Russians don’t have enough precision weapons.
@Yellow Submarine was correct. Trenches are the natural and inevitable consequence of a static front.
The first thing that infantry does when they capture a position and have orders to hold it is they begin to dig in because earth provides the best defense against kinetic threats. Concrete can fragment and be as dangerous as a shrapnel. Hard surfaces redirect shockwave without significant loss of energy. Earth is best - it absorbs energy and doesn't fragment.
A typical trench - approximately 2m high, 1,5m wide - allows an average infantryman crouching by one wall to take up less than a quarter of the cross-section - approximately 0,7m by 0,6m. If you sketch out the geometry for the trench and how airburst detonation affects it you can see how narrow the effective zone of detonation is. If it doesn't detonate directly above the trench then one side provides always better protection. Soldiers take cover on opposite sides of the trench and that increases protection for half of the unit. Trenches are also no longer manned as densely like in WW1 which limits effectiveness of a single shell. Multiple rounds are necessary which reveals position to counter-battery fire. And even if you clear the trench in one section there's an entire network of other trenches supporting it. A "trenchline" is never a single ditch. And if you don't capture position quickly it can be reinforced. Not to mention that trenches can be used to lure in the enemy into a trap.
Trenches are literally the best thing infantry can have in a defensive position.
The best way to deal with trenches is to never let them happen. The second best way is to aggressively shift the battle away from where the trenchlines provide support which requires maneuver - mechanized or airborne.
The problem is that for that you need numbers and firepower and Russia has neither.
Let's make a quick calculation: Let's assume the trenches are 1000km long and each side puts 100 thousand soldiers at the front. This means 100 soldiers per 1km on average - one every 10m. That needs to be multiplied by 2 or 3 for shift rotations which gives 200-300k and then unit rotations which is another 2 or 3 times that resulting in 400 to 900k just to hold the line. That's obviously untenable so the density is likely 3 or 4 times less but probably even less so. But the positions must be held. So the infantry constantly shifts from one place to another depending on how the other side regroups. There's a lot of maneuver happening behind the very static lines but it's the type of maneuver that is not shown on maps.
Now whenever you concentrate your forces you both expose yourself to enemy fire and weaken the defenses which allows the enemy to exploit the opportunity. This causes each side to avoid high concentration which makes breakthrough impossible.
The theater itself also poses additional problems.
1. The depth of the front is uneven.
Russia has approximately 100km of depth at the point of contact in Kherson and between Zaporozhia and Donetsk. That's effectively no depth at all so no major re-grouping can be achieved in the rear. This forces Russia to remain mostly static. The front from Donetsk to Kupyansk has depth but it is poorly supported with infrastructure outside of separatist territory where infrastructure was crucial to holding the area in 2014. These constraints put Russia in a relatively more difficult position because it's located at the perimeter of Ukrainian defenses.
2. The terrain requires different types of mobility.
Operational mobility is best achieved by wheeled vehicles. But the road network in southern and eastern Ukraine in particular is not sufficiently well developed to allow wheeled vehicles to retain tactical mobility. For that tracked vehicles are necessary but they lack operational mobility. Airborne assault could be used to open a section of the front but light infantry will be very vulnerable unless it is immediately reinforced by combined arms push which is limited by the factors above. See the attack at Hostomel in early days of the invasion. Russia doesn't want to repeat that disaster.
Ultimately it is about pressure on global and local scale. As long as both sides can match the other it won't be moved to any significant degree. However due to the constraints of the theater Ukraine has an advantage in the ability to direct concentrated force in different direction from staging grounds. Russia must choose where to concentrate it which allows Ukraine to respond in time. So Russia has to sink more resources to achieve the same effect.
At this point no other technological factor other than mobility to achieve concentration of force matters because concentration of force is decisive at current state of war.
Russia with the third largest air force in the world shouldn't be fighting this type of war.
Every war no matter how advanced comes to that point sooner or later. Every dynamic system that is not chaotic has its emergent equilibrium given enough time. Binary systems are not chaotic. Ukraine vs Russia is a binary system and they're at an equilibrium currently.
Russian made air defenses are too much for Russian aircraft to handle. In fact, no country in Europe would be able to neutralize Ukraine’s GBAD in short order. Only the US has demonstrated capability to do so. Hence why Russia has adopted the attrition strategy of depleting Ukraine’s SAM stockpile over time.
Air power does not win
land wars. It can win sea wars but only because humans are land animals. Not much difference between sea and air but on land humans dig in and hide and that's it for your air power. Ask the Taliban how afraid they are of mighty US air power.
Desert Storm wasn't won by the air campaign. But USAF & aerospace industry spun it that way in DC for sales. Also bombing countries is allows to use violence without putting troops in danger and DC likes that. So the myth lives on because it's beneficial to both sides of the deal.
People are surprised about something that shouldn't be surprising at all because they don't understand war. They understand war propaganda. Wars are not won by jets flying off aircraft carriers to drop smart bombs. They're won by blood, sweat and tears
in the dirt.