One of the main reasons is that this hobby hit the (I do not know what to call it, so Ill throw everything here) "betabrand", "group-buy", "one offs" too dang hard.
You see an engine, its a DCC / Sound / Hyper-realistic switcher and it comes in the rail (Union Pacific, BNSF, CSX, Norfolk Southern, etc) and yard number you want. You have to join the pre-buy list, paid in full upfront. If you miss it (OH, and snipers buy all of them ahead of time to sell later on forums and ebay), well...you are going to pay a lot more later on forums and ebay.
Its nuts. What used to be in stock all the time is now a group buy, "we will never do this again in this paint scheme and road number" production run. Sure, I get it, but it costs a LOT of money to get what you want. I really thought, on my pretty middle-upper class salary that I could relive my childhood and get back into MRR. Yeah, I had to give that up after the realization hit me.. "this small layout is going to cost thousands upon thousands of dollars just in engines and cars, much less scenery".
I have no idea what those MRR companies are thinking, but everyone in it has a LOT of extra money to spend. Even in most forums the line "hard to get new people to join our hobby" is thrown around quite a bit.