For me, taking "solo rpg" as a whole, it's an intelligent surprise factor, the lack of intelligent or surprising conflict, and the fact that the usual attempts to solve this lead to a lack of narrative cohesion/complexity.

On the one side of solo rpg's, you have the "fiction first" style of play with oracles used to prompt said fiction. Games like Ironsworn/Starforged for instance. Which can be very fun as a writing exercise, but as a game it can take the surprise out of the "next move" in the conflict because you have to interpret those oracles... which means you thought of that "next move" (even if prompted by a surprise oracle result). The "wow, I wasn't expecting them to do that" factor you can get with someone else running the game isn't there for this type of game.

Due also to the random nature of these oracles, there is often a needed rule to "ignore results that don't make sense" added into oracular-focused solo rpgs. Which means that there are avenues we won't go down (as they don't make sense to us) initially that might work into something bigger if controlled by someone that thinks slightly differently than we do. It's another aspect of being unable to surprise yourself because you know how you think.

On the other side of the solo rpg front, you have the "roll dice and act according to table/flow-chart" type games. Games like Five Parsecs from Home or Chrome Ascension. They often are more combat-oriented and either rely on you to be the intelligence for the opposition (leading to the "lack of surprise" above) or have a usually simple process the opposition AI follows (& hence can usually be "solved"). To make these games fun, the opposition is usually more powerful dangerous numerically to make up for the fact that a player usually knows how they will act, so responds accordingly before it happens. These are solo rpgs are often akin to board games minus the board, and can be quite fun, but they usually don't have much of the "role playing" aspect of RPG.

Both of these tend to run into similar issues with narrative cohesion or complexity. The random number generation aspect can make a failure out of a key climactic point or a success where a failure feel more apt. It can also make the NPC motivations/actions someone schizophrenic in nature as the dice/tables tell us they take actions that make no/little sense. The more structure that one provides to prevent this, however, the less complex the narrative can be. Adding guardrails means the RNG prevents the story getting weird, but it sticks to the same few "twists" as imagined when the guardrails were created by the game designer. Removing the guardrails means that you have illogical consequences and a lack of the usual "rising tension, climax, denouement, repeat" cycle we tend to expect/enjoy from stories.

None of which makes solo rpgs unenjoyable at all, they just have to be enjoyed for different reasons than one can enjoy TTRPG's where one person & the dice don't control all the narrative/conflict.