One reason for failures when folk run faster than their normal speed is that everything else has to happen faster - especially the planting action. If that has not been overlearned it is likely to fall apart under pressure. One would also have to think that if vaulters increased speed on the runway on any particular jump they might run themselves 'under' and into a poor position for take off. Thirdly increased speed might be associated with decreasing control of other critical elements of the body position at take off - anyone who has been a downhill skiier knows all about that!
While not completely relevant to this discussion - Nikonov's aritcle and table that i refer to in BTB implies that the reason why Bubka and some other former soviet vaulters were able to to accelerate through to take off was the efficiency of their plant -compared with other vaulters who decelerated in the last 5 m before take off.
However the main point i would make - on a theme i have pursued for some time in this forum - is that what really matters is how solidly you hit the pole at the instant of take off and how efficient the energy transfer is at that point. Run up speed - even over the last five metres - is only one part of that package.

Its what you learn after you know it all that counts. John Wooden