It appears to be possible to design non-military ships with drop tanks or detachable external fuel tanks that are more economical than standard designs. Why aren't such ships ubiquitous?
(Note: We are not addressing military ships in this solution; the design of military ships is often strongly influenced by the philosophy of the military establishment rather than by economics.)
We note that both drop tanks and detachable external fuel tanks are mentioned in "canonical" sources, so that while it is possible for the referee to declare them unavailable by fiat, this solution may be unsatisfactory. However, "canonical" references make no mention of regulatory issues associated with starship production and use. This omission is taken advantage of; we use it to provide the reason.
Normally, a standard design is available to a buyer at a reduced price from what would be implied by the specifications. This design does not include provisions for drop tanks or external tanks. We rule that if the buyer wants such provisions, the discount for standard design is no longer available. The price of the ship has just gone up 10%.
We also rule that the jump drives for any ship must be calibrated, and that precalibration is not possible due to variance in each individual ship (i.e., we can't use a standard set of calibration data for a design; each ship must be calibrated individually after construction.) Precalibration data can be preloaded to reduce the time needed to perform the calibration, but the calibration must still be done. This is the most important job done during the last month of construction. However, as a regulatory issue, the Imperium may require the ship to be calibrated separately for each configuration (i.e., no tanks, with external tanks that are carried through jump, and with drop tanks). This adds additional time and cost to the construction process. If recalibration (in each configuration) is required for recertification of the ship as spaceworthy after annual maintenance or major repairs, the costs continue to rise.
In addition, rental, recovery and/or transportation fees can be charged for drop/external tanks; if the tanks are lost or destroyed, the construction costs must be paid as well.
This will probably drive the cost of using external or drop tanks out of the range of most small ships - free traders, far traders, and subsidized merchants. Larger ships will generally have no need for such extra fuel arrangements; they ply fixed routes, doing business at each port along the way, quite possibly with known cargos prearranged, and therefore with generally predictable costs, which wouldn't be reduced by having the additional capability afforded by external/drop tanks.