Why weren’t militaries in the years between WW1 & WW2 more eager to field semi-automatic rifles? Considering the advantage over bolt-action weapons it’s hard to see why more nations weren’t rushing to widely equip their troops with them.

Armies of the interwar period were (for the most part) looking into developing semiautomatic service rifles with the intention of adopting them, but a variety of factors came into play.

Early after WW1, the big issue was the downsizing from the war. The War was over, economies were trying to recover, and, with no new war on the horizon, military budgets were cut. France, who had recognized that semiautomatic rifles were the future and were on their way to adopting a semiauto rifle before war broke out, had found during the war that their 8mm cartridge was a more pressing issue due to its rimmed, heavily tapered design. Keep in mind that France did produce and adopt and issue a little under 100,000 semiautomatic rifles during WW1 (the RSC 1917/1918), but the design had plenty of room for improvement. Thus, France set out on a wholesale rework of its infantry weapons, aimed at taking the lessons of WW1 and applying them to a number of new weapons designed around a new, more modern cartridge. They first set out on a new cartridge, developing a 7.5mm (.30 caliber) straight walled, rimless cartridge in 1924. To go with it, they focused first on a light machinegun - after all, WW1 had shown that mobile platforms for sustained fire were more important than semiautomatic infantry rifles. Though the 1924 machinegun was a good design, experience in the field found that the new cartridge was often mixed up with the similar-looking 8mm Mauser cartridge - something used in large quantities at the time thanks to German war reparations. So the cartridge and gun were reworked in 1929, resulting in the FM 1924/29 machinegun and the 7.5mm French cartridge we know today.

By this point, rifle development efforts were allowed to take priority. Here, France took a three-pronged approach, hoping to avoid the situation they found themselves in during WW1, where adoption was cut short because war broke out just as the decision to adopt was made. Existing stockpiles would be converted to the new round - trials were done with both Lebels and Berthiers, settling on a Berthier conversion - to standardize the army on the new cartridge. All-new rifles would be developed as well in the form of a bolt-action rifle for reservists and second-line troops and a semiautomatic rifle for frontline troops. To streamline production, these rifles would use as much of the same parts and tooling as possible. The result here would be the MAS-36 bolt-action rifle and the MAS-40 semiautomatic rifle. Unfortunately for France, development and production of this whole complement of new guns had dragged out due to much hemming and hawing on the part of military leadership and the general chaos of France during the interwar period. When war finally came to France in 1940, the MAS-40 trials rifles had just left the factory floor, MAS-36s were in use with a handful of units, and Berthier conversions had yet to replace all the rifles firing 8mm Lebel. Ultimately, France was caught midway through adoption of new guns and cartridges, and they weren't able to complete this process until after the end of WW2.

In general, you'll find a similar trend across the belligerents of WW2. The utility of semiautomatic rifles was recognized, but the postwar drawdown precluded rapid development and priority was given to weapons other than infantry rifles like light machineguns and submachineguns. Effectively, the marginal benefit of a semiautomatic rifle was considered to be less than the benefit of introducing other automatic weapons. By the time armies realized war was imminent, it was too late to ramp up production to universally adopt a semiautomatic rifle.