The Imperial Starfleet found the Lancer-class too expensive for full fleet deployment. A few frigates made it into various fleets, but most admirals preferred to use, and subsequently lose, their TIE starfighters as anti-starfighter options. As a result, most Lancer-class frigates, like smaller ships before them, were assigned to planetary defense and rear guard operations.
That's probably what I would say, as well, considering it mentions "full fleet deployment". If the Lancer was actually too expensive overall, it would say something to the effect of mass-production.It costs a mere 4.76m - less than a DP20. Only a week of consumables though, so I'm guessing that's where the expenses come in.
You'd think full fleet deployment would handle some sort of supply ship, or be able to transfer consumables from the larger ships, in order to sustain the Lancer. Guess it was just too much work for old-school Imperial admirals when they already had TIE Fighter screens available as part of their complement.
And whether or not the Lancer may be expensive overall, the reason behind it being cost-effective is it's ability to render starfighter attacks almost useless. I'd say preventing a K-wing bombing run on an Imperial-class's bridge, and saving the ship, would be worth having a Lancer on hand.