It's not simply a matter of availability. If a 1.544 Mbps circuit gets the job done then why pay more for, say, a 10 Mbps Metro Ethernet even if it's only, let's say, 20% more. 20% more on bandwidth you don't really need isn't saving anything even if the 10 Mbps Metro Ethernet is a better "value" than the 1.544 Mbps T1. This is changing all the time. As Ethernet becomes more ubiquitous and the price continues to fall eventually T1s will mostly be completely phased out. I guess if you're installing a brand new circuit and the carrier tells you that they'll install 2 Mbps Ethernet service for roughly the same price as a T1 then it would make sense to get Ethernet since it's easier to scale the service up than it is to go to multilink if you need more bandwidth. However, if you have a foot print of thousands or tens of thousands of sites with T1s and T1 speed is all you really need then swapping out T1 for Ethernet doesn't make much sense due to OpEx hit you'll take making the change.
In addition to Steven's response, it also depends on what's available in your area and what you are looking for in terms of service.
Standard ADSL connections can give you greater speed than a T1 leased line, but the support SLAs for failures on DSL/ADSL are very loose (from what I recall in the UK 3 years ago it was a 24 working hour SLA for a line outage on DSL) whereas the SLAs for leased lines are much better (normally 4-8 hours for a service outage). So while you're not paying for speed, you are paying for uptime with leased lines.