Because they need a niche. When everyone else was falling FR and U2 down the "unbundling" route they did that. When they see LS make hay in their former backyard they suddenly wanted to do the "holiday" thing. Now TOM and especially TCX are doing well on long haul, they fancy a bit of that.
Shame they cancelled those 787s. They would have had a good head start. The short-haul model isn't looking so clever now with Sterling and Euro being close to parity (or worse) for the foreseeable future. TOM FR, U2 and TCX can move aircraft around Europe - MON cannot.