I can relate to your problem as it's something that I faced most of my career. At my peak my squat was 50 or so pounds above my deadlift. I think that this is not uncommon for those who do a lot of olympic lifting because our deadlift style is similar to how we do a clean first pull: low hip start, keeping a stable torso angle/chest up during the first pull. Which is different to a pure powerlifting deadlift in which the hips start a bit higher and the torso is bent forward a bit more too. The olympic-style deadlift is more of a quads movement and the powerlifting a hips/lower back one.
So it is likely that you are very quads-dominant, either through your training style/design or your body structure (longer torso/shorter limbs?).
The problems I see as the possible cause are:
Proportionally weaker posterior chain vs. anterior one. Specifically hams and lower back weaker than quads.
Disadvantageous levers. If you have short arms and a long torso (like I do) the deadlift will be harder because you will not be in super strong position from the start and will have to pull the bar for longer than if you had longer arms.
Strength curve favoring strength above the knees. VERY common for those who do a lot of olympic lifting. In olympic lifting we are basically producing a very high level of force only once we pass the knees. When you do a lot of pulling with proportionally light weights (your olympic lifts will be light relative to your deadlift) you do not overload the initial part of the lift (from floor to knees) BUT you do overload the second part because you accelerate the weight maximally (Force = Mass x Acceleration... even though the weight is still "light" since you are accelerating it as much as possible you produce a high level of force). If you spend a lot of time doing the olympic lifts your nervous system will thus "learn" to produce a lot of force only once you pass the knees. And as a result your deadlift will be weak because you can't overcome inertia off the floor. In other words you do not have a lot of strength reserve to break the barbell off of the floor.
No much you can do about issue no.2, we are all born with the levers we have.
For problem no.1 the key is to select assistance exercises targeting the posterior chain. Arched back goodmorning (focus on feeling the hamstrings getting loaded), romanian deadlifts, glute-ham raises, back extensions and even wide stance box squats can be of great help here. This can be boring work but if you want to solve a weakness you must orient most of your assistance to correct that issue.
For problem no.3 the key is to emphasize the first pull more often to strengthen it more. You don't want to deadlift often, sadly this is probably the most important thing to do. From your lifts it seems like you have plenty of strength to deadlift a lot more, but you need to learn to apply that strength on the deadlift itself. And the only way to do that is to practice it often. No need to overload it every day, you can go heavy only once a week, but I would suggest doing some deadlifting at least 3x a week, even if it's just 2-3 work sets.
I also suggest using variations in which the first pull is made more challenging to counterbalance the fact that you are overdeveloping the second phase of the pull with your olympic lifts. Some options include:
- Deadlift from a deficit (standing on a 2-3" platform to increase the range of motion.
- Snatch-grip deadlifts
- Paused deadlift (pausing for 2-3 seconds as soon as the barbell is off the floor)
- Double first pull deadlifts: doing the first pull twice per rep... lift from floor to knees, bring back down, lift completely, this is ONE rep
Another exercise that I like is half SQUATS from pins... put the bar on the safety pins at a height where, once the bar is on your back, you have a knee angle and hip position similar to the start of your deadlift (this likely means that your torso will lean a bit more forward than during your regular squats). From that position lift the barbell and lower it back down to the pins very slowly. Start every repetition from a deadstart from the pins.