psychoak and Stress hit the nail on the head - the producers of TNG were pressed for budget and few ships were made because every new ship design had to be created the "old way," which involved having artists sketch it on paper, mold it in (clay I think?), and then move on to making molds, molding it in plastic, wiring it up with lights, and painting it. The average cost of special effects for TNG was $200k/episode, but that wasn't much given that computerized special effects weren't available.
That ship creation process took time, which is why (as Stress notes), the only Romulan models were the D'Deridex (TNG), a scout (TNG), a shuttle (DS9), the Norexan (TNG: Nemesis), and the stupid Scimitar (TNG: Nemesis). The truly disappointing part is that by the last three or so seasons of DS9, the models were all computer-generated (there never was an Akira class model, for example), so they had few excuses for not making them. Instead, the just spammed D'Deridex warbirds and had them exploding all the time as though they were Miranda Class (go look at the big battles on DS9 (all on Netflix) and you'll see that no Galaxy class ever explode and very few Fed cruisers (one or two Excelsior class)).
As Stress notes, the D'Deridex was supposed to be a battleship, dwarfing the Galaxy Class. Per the explanations offered by the TNG storylines, the Romulans effectively abandoned their borders after their war with the Klingons (the one where the Enterprise-C was destroyed and established a formal alliance between the Klingon Empire and Federation). They were thus a "new" old villain for TNG, and they only appeared when they had the upper hand. That's why I think the "Preator's Talon" ability in SOA2 is so appropriate for them - it fits with the Romulans' canonical strategies for battle.
I'm also glad to see nothing "Reman." Remans were only added so that there was an excuse for a political schism in the Romulan Empire for TNG: Nemesis, and they were a stupid addition at that. If you look up the ostensible origins of Remans on Memory Alpha, the Nemesis producers basically wanted a super badass species that looked like vampires - apocryphally, they were established as the "telepathic" Romulans who were enslaved and forced to work in mines. #$*&-ing stupid. If you wanted super badass Romulans, why not just have telepathic Romulan commandos who don't look like a second-rate Buffy villain?