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Well first of all let's consider what "executive class" means. It just means the passengers are not packed together like sardines, on a bus. This is not an aircraft. The cost of moving that bus 150 miles is not that high whether you pack 30 people on the bus or 60. So we are not talking USD $500 fares here, I am sure.
Second, the poorest of the poor stay local, and can't afford to take much time away from whatever they do for survival, and would have little need for long distance bus. If someone is traveling long distance, they probably moved to "the big city" and therefore have moved up a rung or two economically. They may not be fabulously wealthy but they can afford a fast, comfortable ride to visit mom.
Third, just because a "luxury bus" is the most prominent way to go from City A to City B does not mean lower income people are not taking less glamourous transport that flies under the radar. Here in my part of north america, there's a whole system of unofficial local buses that operates in parallel to the "real" buses operated by state-owned transit authorities. They are crowded, and not luxurious, and the fare is paid in cash to a human fare collector. You can't really find much on the route system online, and there is definitely no app or anything fancy like that, but the locals know about it, and use it extensively. Sometimes they call it the "chicken bus" though it is not one bus. I have seen similar things in developing countries though I didn't explore those systems much, not knowing the language.