Vendors Pre-order policies?

Maybe someone can explain this to me...why do some vendors take pre-orders on items and don't fulfil them in a timely manner??!?!

Here is my story:

I place an order with DragonUSA Online for the Bronco M24 Chaffee, 2 sets of Bronco Stuart track, and the Zvevda T-90 kit. I understand that the T-90 won't be out till end of January and the Chaffee till the start of the month (I was placing my order in December at the time)

In the meantime while waiting for these items, I place another order for the Bronco Buffalo and for the new Russian Sherman book at the start of January. I understand that the Buffalo won't be out till the end of Feb or so

DragonUSA Online got the Chaffee and tracks in the middle of January. I sent them an email last week asking for my orders to be filled, expecting the Chaffee, Tracks and book to be sent. I got a reply back from them saying they would be getting getting in their Zvevda shipment in last Wedensday or Thursday and would ship it then.

That came and went....so I get a shipping notification today from DragonUSA about my order...and the only thing they are shipping to me is the Zvevda T-90 (which just came on Tuesday) and the Bronco tracks! The Chaffee is out of stock...which is pretty madding since I pre-ordered mine back in December! I know they want to keep shipping costs down, but all they did was make it quite possible that they are going to have to make 3-4 different shipments to me, when they could cut it down to two if they kept my pre-order!

Is it too much to ask to fill your pre-order stock first, before going out of stock on new orders? I know other vendors where it becomes a viscous circle...you say order 2 items, one item goes out of stock, then that item comes back into stock then the item that was originally there is now Out of Stock! I just don't get why you would do that.

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