It's all about the dificult proposition of maintaining profitability in a manufacturing situation with low volume sales numbers. Vinyl, while growing in popularity, is still a small, insignificant niche of the music industry. It's a lot more complicated to manufacture a vinyl release than you'd imagine. Pressing plants have been closing and it's harder to get production scheduled in the remaining quality facilities. Also, a record label has to get titles pressed in reasonably high numbers to gain some economy of scale in manufacturing so they can make a profit. Hence, demand may have to build for some time before it makes economic sense to reissue.
Vinyl question
I have just started my vinyl collection (bought my first TT this month). Please explain why is it hard to find certain quality recordings in Vinyl. I don't understand how certain releases get "sold out". Is not simple to continue production of a particular recording if the demand is there....or is the demand not as strong as I think it is.