Extremely soft vegetables like ripe tomatoes are difficult to julienne because their delicate structure makes it hard to create uniform, thin strips; they tend to squish and fall apart.
Julienning requires a certain level of firmness in the vegetable to maintain its shape during the cutting process. The technique involves slicing the vegetable into thin planks, stacking those planks, and then cutting them into matchstick-like strips. With very soft vegetables like ripe tomatoes, the internal structure is weakened. The high water content and delicate cell walls mean that applying pressure to create the initial planks often results in the tomato collapsing or becoming pulpy. Even if you manage to get planks, stacking and slicing them into julienne strips becomes nearly impossible, as the tomato will simply mush under the knife. The result is a messy, uneven cut that doesn't resemble a julienne at all. Other cutting techniques, like dicing or chopping, are much more suitable for soft vegetables.
If you absolutely need to julienne a slightly soft vegetable, try chilling it thoroughly in the refrigerator beforehand. The cold temperature can firm up the vegetable's structure just enough to make it slightly easier to handle and cut. However, for extremely soft vegetables, it's best to choose a different cutting method.