There are several reasons why we wouldn't accept multiple strains of crypto-currency. Here are the main ones:
* Losing confidence in the concept as a whole is the primary reason. If there can be two, why not ten? If they can come and go on a whim, then how solid can this magic internet money really be? We don't want to be playing Frogger with our money (the old arcade game where you jump from one sinking log to another).
* It's counter-intuitive, but giving the customer more payment options reduces sales. It makes your sales process more complex, requires the customer to make more decisions, and thus reduces overall conversion rate.
* All the back-end overhead to manage not one, but two, different crypo-currencies would add additional expense. The exchange rates would likely be different, we'd have to deal with twice the maintenance and technical upkeep, our accounting would be more complicated, we'd lose economies of scale if we need to convert currencies, etc. Do we pay our employees in Bitcoin-A or Bitcoin-B? If some want A and others want B, but we have different amounts of each, then what? All the extra support that goes into accepting the current Bitcoin is enough of a cost given its extremely low volume as it is.
The only one of those that might hold water is #1.
Point #2 is patently false. Yes, it sounds counter-intuitive because it's not true. Giving the customer more payment options always increases sales, because it expands who can be a customer. If some product is desired at all then having maximum options to pay for it is a bonus/benefit, not a drawback.
Point #3 is understandable, but still arguable. It depends who you are, or, rather, how technical you are, and what technology is available. The back-end overhead to manage one crypto-currency or multiple crypto-currencies is not all that different because everything is software. You don't have to re-secure a server to hold more than one crypto-currency. The server is either secure or it's not. Conversion of currencies would be more the issue, but even that could be handled smoothly as, again, everything is software; it's just bits, bits which can move around the world in seconds for free. Giving employees choice of payment? Again, more choice is always better for some things, and money is one of those things. Lastly, about the presently low volume, that isn't expected to always be the case.