Because
a) The Irish need overseas opponents of a high quality on a regular basis and the Welsh fit the bill better than anyone and finding replacements will be very difficult.
b) As someone who would like Welsh rugby to do well, with a small vested interest in its performance, I honestly think you're better off not going. And yes, there's probably some bias to that, but I'm trying very hard not to have any and looking at it as an Englishman and an Ulster fan.
For one thing, it means total civil war. The WRU will not wear it lightly and there will be a huge scramble over players for each set of Welsh pro sides. That weakens both sides. It's likely to drive more players to France with the intent of waiting it out. I'm slightly behind the times in possession of the facts here - is there still going to be a legal battle first? has there been a second offer yet? But, frankly, I think virtually any compromise between region and union will be a good thing - and I don't see England happening on that.
Nor is it a solid long term proposition. You cannot have a league that size and have Europe and I feel reasonably safe the English clubs would rather have Europe. It pays more money. The RFU want it. It opens up a higher level of competition. You guys are a second choice. Should compromise on a European tournament happen this season I rather imagine the invitation to join would be withdrawn. If not, participation probably ends at the end of those five years.
While I see how the current situation with your union is highly undesirable and forcing the regions' hand, I see this as a move from working with people who see you as long term partners to working with people who quite possibly don't. That is not a sensible move. At the same time you are putting yourselves foursquare against one of the IRB's governing principles which, to me, seems a rather messy way of doing business.
Yes, there's some friction. Yes, it's annoying to be blamed for a Welsh lack of interest - but by lack of interest, what is accurately meant is (or should be) the Welsh lack of interest in the regions, not the regions lack of interest in the Pro12. But I'd rather the Welsh stayed - and that's both for Ulster's sake and Wales' sake... and England's sake too actually.