I'm far from an expert in Portuguese history, but broadly speaking I'd agree that it doesn't get the same level of attention as Spain does. The Salazar dictatorship tends to pop up in mainstream European historiography in two contexts. First, comparative studies of European fascisms - how far the regime should be seen as fascist, and how it fits into broader conceptual theories of fascism. Second, studies dealing with transitions from dictatorship to democracy, where Portugal is seen as a relatively successful model. You'll note of course that these two categories also apply to Spain - so taken together with Spain's larger size, it's easy to see why Portugal might struggle to escape the shadow of its larger neighbour's history.

However, setting aside relative size and historiographical advantage, there is one other big factor: the Spanish Civil War of 1936-9, the conflict that led to the establishment of Franco's dictatorship. The civil war was a major international crisis, not only due to its deleterious effect on Spain but also because it saw the participation of several major world powers, including Nazi Germany, Fascist Italy and the Soviet Union, becoming a key (perhaps the key) ideological flashpoint ahead of the Second World War. As such, it is a key reference point in most histories of the origins of the Second World War (which is quite a lot of histories), as well as a quite considerable interest in the conflict in its own right.

This historical interest was driven not only by the war's geopolitical significance, but also the impact it had on a key generation of European historians and intellectuals. For anyone at all politically minded in the 1930s, it was just about impossible to escape the Spanish Civil War as a burning, vital issue. Writers and artists lined up to take sides (generally pro-Republican, or anti-Franco), and a significant number went to Spain to observe or even fight directly, such as George Orwell (who fought) or Ernest Hemingway (who didn't). Tens of thousands of less literarily talented people also journeyed to Spain to take part in the conflict, and the large-scale involvement of so many foreign nationals also did a great deal to stimulate subsequent interest in the war. For the left in particular, the struggle took on mythic proportions - particularly after disillusionment with the USSR set in after 1956, the doomed, noble attempt to save the Spanish Republic became an increasingly cherished memory. Even today, history writing on the subject can be highly emotionally charged. The best parallel I can think of is the Vietnam War - a regional (even internal) conflict that was elevated by its ideological and international context into a generation-defining political issue across much of the world.

In contrast, Portugal was never at the epicentre of a comparable historical flashpoint. The Portuguese wars of decolonisation were bloody and protracted for sure, and have attracted their own share of historical interest, but ultimately were not the same focal point for international attention at the time or since. Histories of the Salazar regime certainly exist - as mentioned above, I'm very far from an expert but the works of António Costa Pinto (in English) are well-regarded.