Questions asking why people in the past didn't do something we now take for granted are difficult to answer. Typically, the answer is either "what they did do wasn't as bad/inefficient as you think it was," or "they just didn't have the ability to do what we do now."

Medieval shoes were made shaped for individual sides of the body, and the straight last (shoemaking form) was an invention of the Early Modern era. For instance, in the Museum of London's Shoes and Pattens, it is noted that "all shoes in the present collection, unlike those from 12th-century Middelburg described by Groenman-van Waateringe, can be easily identified as either left or right; similarly, a right-foot boot and a left-foot last are reported from Anglo-Scandinavian York." Even as late as Shakespeare's King John, a character says that

I saw a smith stand with his hammer, thus,

The whilst his iron did on the anvil cool,

With open mouth swallowing a tailor's news;

Who, with his shears and measure in his hand,

Standing on slippers, which his nimble haste

Had falsely thrust upon contrary feet

18th century commentators like Dr. Johnson actually took this as Shakespeare making a mistake, because they were so used to straight-lasted shoes being the norm that "putting your shoes on the wrong feet" simply didn't exist as a concept.

And at the same time, straight-lasted shoes continued to be the norm through much of the 19th century and were still made going into the 20th, although when they were carried by retailers in the latter it seems that it was for a niche market, with stores carrying only one line of straight-lasted shoes. Today, they appear to be widely recommended for severe overpronators (I will have to look into that) and aren't widely available, but do exist.

Curiously, there is not much scholarly discussion of why people switched to curved/crooked lasts, which could help to explain the answer to your question. Historian Julie Swann attributes the change to straight lasts to the introduction of heeled shoes because it was more difficult to make lasts shaped for both heels and sides, and the return to flats (which she puts around 1800) spurred a return to curved ones, but I'm only coming to this reference from another source that quotes her and not the original. It's therefore difficult to know exactly where she's pulling this from, but to me it seems like drawing a causation from a correlation. It's not as though all flat 19th century shoes were made on curved lasts! The flattest shoes of all, women's day or evening slippers, were typically made in straight pairs, and the boots being worn by soldiers in the British army ca. 1800, who were instructed to alternate their shoes each day in order to make them wear slowly and evenly, were also certainly flat; the earliest reference I'm aware of with a firm recommendation for curved lasts is the 1824 Art de la Chaussure, a manual on shoemaking. Other historians have put the main changeover to the 1840s and 1850s (which fits better with my experience of examining actual historical shoes, as well as the note in the 1847 Book of the Feet that "straight lasts ... notwithstanding what many persons say to the contrary, are decidedly inferior to a well-formed right and left pair;" I would still reiterate that at least most women's shoes through the 1870s, if not the 1880s, show no evidence of having been made for specific feet) and attribute it to mechanical advances in shoemaking: machines were developed that did away with various processes normally done by hand, which could allow more time to be spent in other ways, i.e. making symmetrical rather than identical pairs of shoes. But, that being said, as you point out in your question, the wealthy wore straight-lasted shoes during their period of ascendance - it's not just a question of time vs. price, because elites during the Early Modern era were willing to spend a lot of money on their clothing in order to get the best materials and have a lot of man-hours put into individual garments.

So, um, the best I can really do is to say that they wore straight-lasted shoes because the sole shape wasn't seen as that big of a deal. As we've seen, for some feet a straight-lasted shoe is actually beneficial; according to The Book of the Feet, curved lasts could be problematic for all wearers by giving too little space to the toes. Without padding or contouring, the sole was going to lead to foot problems and discomfort anyway. (And even with curved lasts and padding and contouring, a great many people have foot problems and discomfort today). The switch from straight to curved lasts was not exactly a discernible step up on the "tech tree".