Why would anyone not choose the cheapest available option?
They wouldn't -- so most booking sites don't bother showing all the options, and just present the cheapest price for each service class that's still available.
However, the data format that airlines use to inform ticket sellers happens to work in terms of fare classes. The travel agency's computer doesn't get told, "a seat on XY1234 costs $120, but a seat on XY1236 two hours later costs $155". It gets told, "XY1234 can be booked in fare class Q, P, T, E, S, and XY1236 can be booked in fare class P, T, E, S". It then looks up in a different set of tables what a ticket between such-and-such cities on such-and-such fare class costs.
Usually, the customer-facing front-end will translate away this complexity and, say, not show the more expensive options there would be no reason to buy. The one you're looking at apparently doesn't (fully) do this -- which might be because it is cheaply or quickly made, or out of a principled ambition to present as much information as possible to the customer, whether practically useful or not.
(Travel agencies need to know all of the available fare classes, because connecting flights are priced by fare components that may include several flights but has to be the same fare class throughout. So if you want to fly on XY1234 from AAA to BBB and then connect to XY543 from BBB to CCC, and it turns out that the lowest fare class available on XY543 is P, then you need to find a P fare from AAA to CCC, and it won't help you that XY1234 still has Q seats bookable. This complication is usually not shown to consumers either).