I got two sponsors about 10 years ago. A lot of things happened in the meantime in the world of climbing but here are a few sure things:
Is it harder to get sponsors as a person that only boulders?
No, it is not, and as of now (2019), it might even be easier. Bouldering experienced a huge boost in the past years, a much larger public than before has its eyes on it and being able to play big in this scene is something sponsors are very interested in.
what ratings most athletes can climb when they become professional rock climbers.
Having a sponsor is a first step towards being a professional rock climber but not at all equal to being one! That being said, you should present all your skills as an athlete,
not only your highest grade. Getting sponsored means advertising the products of a brand in exchange for those (and possibly money). Any possibility you have to attract attention
for your sponsor is a relevant argument to present:
Obviously the better you climb, the higher your chances are. But:
can help too. That may be technically (e.g. trad, bouldering, multi-pitch, drytooling, FA, competition) or geographically (climbing in a region where a given brand isn't known yet, advertising a new climbing spot).
is also very relevant. Climbing 5.13 with 15 might sound (even) more impressive than 5.14 with 25.
Contacts with magazines, climbing forums, online platforms and social media followers can be a heavy argument. Climbing just "okay" but being known and seen by many, you'll have better chances
than a much better climbing noone knows.
Finally, don't forget that sponsorship is a job (you provide a service to your employer and get a compensation for it) and applying for it works like for any other job: keep sending applications and wait for one positive answer.