A fews reason for this:
1) Manoeuvrability. In BL cavalry feel very manoeuvrable at high speed but this doesn't seem to improve as speed decreases, which feels counter-intuitive. The result is cav are encouraged to do high speed hit and runs, easily turning right angles at top speed with couches, but struggling to confidently slow down, turn, and get stuck in on fights for a longer period of time.
2) Fairly high threshold required to bumpstab. This is easier than earlier in development but it still feels like you need to be going a good 50% of speed at least to reliably bump stab. Otherwise you simply move the opponent around in an awkward manner while they damage your horse or, frequently, accidentally hit your foot doing a decent amount of damage. Again, this encourages hit and run instead as it's simply safer and more damaging, but the payoff is the archer is less disrupted (unless dead!)
3) You can't dismount on archer because they can simply take your horse. In Warband for example, if you were winning but your horse was low, or an archer was in a really threatening spot difficult for cav to reach, you could ride up, dismount on them, and disrupt them that way by keeping them in melee while the rest of the team fought.
Some solutions:
A) Tweak the turning stats for horses to be slightly easier at lower speeds and slightly harder at high speeds, should stop the race car drifting couches and encourage sticking around in fights.
B) Very slighty reduce the threshold for bumpstabs, and if possible completely stop or heavily reduce the moving people around aspect of cav. IMO Warband was broken when it came to bumpstabs but it did provide a balancing aspect to archers, so there should be a middleground.