No one actually believes in Scripture Alone on a personal scale. Things can be revealed to individuals that aren't revealed to the community. The seven thunders in Revelation is an example where something was personally revealed to John the Revelator, but he was instructed not to include it into scripture. It is possible for things to be revealed outside of scripture and for that thing to be true. The Holy Spirit leads us to the understanding we need in our lives at the time we need it.
But Sola Scriptural is a comment on communal knowledge, not personal knowledge. And even Martin Luther included early church tradition into the movement that he started. What Luther called Sola Scriptura wasn't a true Sola Scriptura by name.
Sola Scriptura is just a starting point. From Sola Scriptura we can rule out things that are impossible or otherwise incompatible. We can observe what is a possible interpretation from scripture first. Sola Scriptura is in essence the philosophy that in the event of a contradiction between scripture and established tradition, that scripture takes precedence.
Luther's big push with catholic Protestantism and Sola Scriptura was 'in protest' of the established traditions of the Roman Catholic Church at the time, including indulgences, etc.
Luther opined that the traditions of the RCC should not be interpreted to have the same authority as scripture and that everyone has the right to reject tradition but not scripture.
A tradition represents a school of thought, and those schools of thought may be specific to some people and true for those people while at untrue for other people.
Rom 14:5 and Rom 14:14 are great examples that point to the idea that people may and will have individual requirements for how they engage the world that may contradict traditions but be consistent with scripture.
If an ethnic group had an alergy to shellfish, it would likely be 'unclean' to them and should not be consumed. For another ethnic group, it could be the case that shellfish is a staple of their diet.
Or dairy products such as cows' milk where specific ethnicities don't lose lactose tolerance into adulthood. Some ethnicities can't handle milk products as adults. To one group dairy might be clean, to another it might be unclean. It could be the case that a church determines for its practitioners that drinking milk as an adult is unclean because 100% of the congregation at the time finds it to be unclean. But there is something lost in that tradition that can make it contradictory to scripture. That tradition may be the correct approach for some people but not for others.
The topic is hard to explain, but when look at the fact that Greek Catholic and Roman Catholic both hold their traditional teachings to have equal authority to scripture, we can unintentionally cause a collision between tradition and scripture when applied to new circumstances.
That was more or less what Jesus criticized the Pharisees' traditions for doing: contradicting the true meaning behind scripture. And specifically contradicting the spirit of scripture by using a tradition based on the letter of law.
Sola Scriptura has its merits, but scripture as a primary source of authority (instead of only) is more common.