While monero is very cool, it has the same scalibility issues as bitcoin i believe, but to a worse extent as the transactions are much larger due to how the ring signatures work. I know they're working on this but it's not a straight upgrade, and last time I checked monero fees were quite high compared to other alt-coins with similar use.
Network effect and name recognition, the same reasons that Bitcoin hasn't been supplanted by Monero or Zcash or any of the other dozens of coins that do virtually everything better than Bitcoin.
The really interesting thing is that the original plan for Bitcoin included many of the same programmable-money features as Ethereum. The instructions are still in the bytecode but I believe they were disabled in one of the patches, except for a handful of instructions that implement the basic "send money from A to B" type functionality.
My impression is these were supposed to be enabled/used at some future date, but this is just one more thing that's gotten bogged down by the infighting and politics of the project. Much like blocksize - 1 MB was just a number picked to avoid transaction spamming blowing things up in the early days, not a divinely inspired number intended to be used for all time. But the project is so stagnant and calified that they can't handle even the smallest day-to-day tasks like this. God forbid we change one of Satoshi's sacred constants.