I have a pretty good assumption as to why bottom side is often left open and that's typically because of a couple of major factors which stems from RIOT trying to open up bottom lane to other roles. While it sounded nice to see other roles being allowed to play in that lane, it isn't exactly fairly paired or positioned well enough to have more roles. It isn't like bottom lane isn't already opened up to Catchers, Enchanters, Burst, Vanguard and Warden, but now they're opened to Artillery, Battle Mage, Slayers, and worst of all, Specialists.
All that aside, lets talk about 3 major problems that I can see what the common player would have. The first problems I see stems from the win rate percentages. Out of 20 choices (Yasuo and Senna included) there are 12 champions above 50%, 8 of them are classified as S and +S tier in this current meta, and 5 of them have a high play rate above 10% in platinum. This kind of information can scare anyone especially since banning these picks from the "meta choice" is no longer an option- unless your entire team is in an agreement. Additionally, counters to these S and S+ tier picks are actually poorly ranked due to their play ability within the current meta being quite poor itself.
The second assumption I have is the intensity and type of communication the lane must have when relying on your teammates and this is because of the focus that has been placed on the dragon pit. A lot of times the pressure found bottom side will always be because of dragon pit, but this time around it is too much. Teleport has been seen to be a little more mandatory compared to heal or exhaust for bot lane and top side will more often be there for the first dragon. Tower plating just gives away too much gold to sacrifice, so if you win lane really hard your hand it sort of forced to stay there until the tower plating goes down in order to rotate. After all, free farm is already enough to come back from- but now turret plating?
3rd and lastly, farm has become a bit too hard to last hit. Vladimir has often been known for having the worst auto based damage to farm with and sadly it is comparable with consistently missing 2% of total farm 15 minuets in. This wreaks and says something about how important the first kill gold is and why it is often oriented as a one sided stomp. If you don't play something that works well with last hitting then saving up your gold to do last hitting damage will severely cost you and ultimately remove your spike. This time around now, Critical striking marksmen are no longer the tide turning choice and that my friend, should definitely be lingering in RIOT's mind.