big 4 tax practices are much smaller than audit practices. they account for a significantly smaller percentage of total revenue than audit. smaller group = less people = less new hires.

also, most people are attracted to audit over tax because audit is much broader. usually the people who go for tax have some sort of connection to it and a reason for why the want to do it (i.e. they have an interest in law or maybe they have a family member/close friend who works in tax, etc.)

if you're set on working in the big 4, take whatever job you can get (audit, tax, advisory) and if you don't like it, you can typically switch after year 1.