Having loads of individual ISDN connections to one place is all very well if you want to have lots of phone lines, but what if you just want more bandwidth? Primary rate ISDN, aka T1 or E1 lets you group a fixed number of ISDN lines into one wire. This lessens the problem to an extent, but at most you can only get 1.5 megabits per second (T1) out of such a service.
This isn't very satisfactory if you want say, 10 megabits. Sure, you can go to the next level in the hierachy, T2, but that's only 6 Mb/s. If you really want 10 Mb/s, you'll have to get a T3, which is 45 megabits! That'd break any normal person at the rate that bandwidth-bandits charge.
BISDN was developed in an attempt to be able to allocate arbitrary bandwidths to a customer yet retaining the SDH nature of the network. ATM is the technology that ended up being used to implement it. ATM does do the job, but it's not clear how well.
Cisco decided to go one step further when it created PoS, which delighted people just interested in plain, old, raw broadband.