For the same reasons I'd book with any other carrier.
Unlike some airlines, Monarch is protected through the ATOL scheme, so you're in no danger of losing your money. If I've booked using my credit card (and taken the hit on the CC fee) my card company is responsible for refunding me. Finally, if the CAA are happy, then so am I.
I could book with a legacy carrier, and find myself stranded through industrial action; or I could book with another large, infamous lo-co carrier, not necessarily HQ-ed in UK, and find I'm left high and dry as a result of a short notice cancellation. There's risk in everything I do, and to every pound I spend.