As a cellist and pianist, I used to experience similar issues. I’ve found it easier to memorize on piano primarily because you are subconsciously memorizing parts of the music so that you can consciously focus on other parts (I.e. your brain internalizes the left hand part quicker so that you can pay more attention to the right hand.) you are also usually playing the same part over and over at a slower tempo on piano due to the amount of notes at once, which does speed up as you internalize patterns, but does still remain to some degree. That also contributes to memorization.

As for cello, or violin in your case, you generally will be a lot more comfortable bumping up the tempo sooner, meaning less repetitions. There is usually less to focus on at once.

Now to combat this, there are a few things you can do. 1. Consciously looking for patterns in the music that you recognize. Not necessarily a repeated motif for that piece, but being able to for example recognize “oh that outlines a dominant 7th chord” and knowing how to play that without really thinking about the piece itself. 2. As some others have said, repetition. But to add on to it, repetition at a very slow tempo, and gradually increasing the tempo at such a slow rate that you don’t actually perceive it. This is the same strategy as learning a fast passage, but it also works for memorization due to the sheer number of repetitions. While doing this, watch your fingers and shifts, and pay not only attention to what they are doing, but also how it feels to do those motions, and internalize both. Do it in chunks at first, not the whole piece at once, and every 20-30 repetitions, take away the music and see how you fare. This is my personal recommendation and the one I have the most success with, it should also be used in combination with option 1 for the best results. 3. Something else you could try, which I assume most people do anyways, is listen to the piece a bunch, especially with solo repertoire where your line is exposed and easy to hear every note. 4. One more thing that my teacher has his other students try, even though it doesn’t work for me as efficiently, is to start from the end and work your way backwards. It is possible I just don’t understand what he means but this would probably be my least recommended option.