Hie, there are so many reasons, one could pretty much summarise it with "everything" 🙂
Their base models comes on straps, not bracelet, and have screwed lugbars. (Have a look at vintage mil divers, a lot of fixed lugbars, with good reason). 1315 with 3 barrels, specifically made for diving, excellently finished, a luminated bezel that works, curved for viewing angles, years and years before anyone else.
Top notch finished dial, hands, indices/numerals and lume without compromising diving qualities. The date is placed and coloured to be ignored during diving. The 45 is sized to be optimal for diving and they makes no excuse for it, because they are right.
They have prioritised grip by a great ratio between bezel height and total thickness where others slims down the bezel. They don't add Helium valves to watches not made for diving that requires it, and they do for the watches that are made for it. X-Fathoms, Gombessa and 500 Fathoms.
None of their divers have excessive wr ratings. The 500 Fathoms rating corresponds to the physiological limit of humans plus a safety margin. X-Fathoms and Gombessa both sticks to 300m. They are still at it, improving diving capabilities.
Another thing they have done is to address wearability by providing a series made for that in order not to compromise their focused divers. By Bathyscapes down to 38mm. Capable they are, but the priority is different and more in line with what others do.
As high level as Fifty Fathoms truly are by tech, quality, looks and finish, they have never compromised them as tools. I belive they were ahead of the rest with the FF "Triology" 🙂
They are simply the reference to compare anything else with.