Outlay mostly, despite most of us being keen for repro's the mould production costs etc are high and then as the case with mombat, when you get them from the states to the uk and that's $100 then the postage of $40 then the inevitable customs which will add about another $40 and of course parcel forces slice of about $12
so suddenly your total for cheap retro tyres becomes 121 quid. The trouble with that for mombat is that suddenly a lack of customers wanting them see's them with a lot of tyres that to now shift will probably be sold either at a loss or..so cheap that the profit would not be worth it at all
As long as you don't mind waiting then you can get originals on occasion for about the same price but there is the trade off, originals could be 20-30 years old, so how good would the tire be.
When it comes to tyres we just have to except that the supply is draining out and we will have to accept alternatives, repros or incredibly high priced nos old tyres