Activity toward the desired substrate sequence is not the only concern in evaluating a restriction enzyme buffer system. In addition, you need to be concerned about so-called "star activity", or activity of the enzyme toward other, non-desired nucleotide sequences.
For example, NEB supplies an enzyme-specific buffer for it's standard EcoRI, even though it lists 100% activity in buffer #2 of NEB's standard buffer series. But at the very bottom of the page, there's this note: "May exhibit star activity in NEBuffer 2.1 or CutSmart Buffer."
Promega has a similar warning about it's "MULTI-CORE" buffer - even though EcoRI has 100% activity in that buffer. The MULTI-CORE buffer is not recommended due to star activity - cleavage at non-standard sites.
It's a little hidden on the product data sheet, but Thermo Scientific does have a mention of star activity in the standard "Tango" buffer, and mentions star activity with overdigestion in the standard "R" buffer. There's no warning about buffer "O", but given EcoRI's propensity toward star activity, I'm guessing this is part of why they recommend a unique buffer for routine usage. Given how frequently EcoRI is used in molecular biology, it's worth using a custom buffer to get the star activity as low as possible.
From canadianer: I contacted Thermo's technical support. Despite the fact that star activity is not listed as a concern for either 2X Tango or Buffer O in the product data for EcoRI, they say that it may actually show some star activity in these universal buffers which is why they continue to supply the unique buffer for this enzyme.