The original reason is that they got offers from each manufacturer which were too good to refuse. First to become a new A-320 customer, then Boeing gave them an extra good deal on more 737s again. Also due to the merger with DBA they already inherited a big 73G order so they might have forgotten about trying to get a single type fleet ever by then.
Their fleet is so big that the synergy advantage of having a single fleet is not so important anymore.
In a few years I guess they'll operate around 75 of each.
nobody has ever died from hard work, but why take the risk?