All bard songs increase with level, and when Kunark came out, Selo's increased as well. But nobody remembered the movement cap. When a bard levelled past 50 and used a Selo's Drum of the March, his speed went past the anti-cracker cap and was reset to zero. This wasn't discovered until just before Kunark ship (surprise, bards weren't well tested), and thus there wasn't time to get a code fix before release. So Selo's was changed to cap at level 50, as a temporary fix until the movement cap could be fixed. That temporary fix still exists today.
From my tests, the cap is set at 24 instrument modifier, for any bard level 50 or beyond (+65% base boost from Selo's), or +156% speed. This same speed should be obtainable by a level 46 bard (61% Selo's boost) with a Drums of the Beast (26 modifier).
Note that faster speeds are supported by the client and the server. GM's move much faster than the bard movement cap. Since there isn't any more processing required to track an object adding 100 units per second as there is an object adding 50 units per second, there aren't any technical reasons for this cap.