From our research, it sounds as if you were just unlucky in your choice of hotel. A call to Main Street Station confirmed your experience and that they seldom permit a Saturday arrival, citing the fact that Saturdays are traditionally busy and tend to sell out, but the hotel doesn't want to sell those rooms separately and then be stuck with a lot of Friday inventory that they can't get rid of because many guests do want to stay for the full weekend or not at all.
Circus Circus said they used to implement a "no Saturday arrival" policy but enforce this much less strictly these days, with visitation having been down in general and increased competition on the Strip. The Tropicana said it depends on how busy the weekend is, while The Artisan and South Point both said they had no restrictions on Saturday check-in. So, if you're looking for Saturday-arrival options, our suggestion is to call around some other properties if your first choice can't or won't accommodate you; there are plenty of other hotels that will.
