There is no one-to-one correspondence between personal holiness and effectiveness in the pulpit. After all, actors (and con artists) pretend to be people they are not all the time.
Likewise, there is no direct connection between personal holiness and the size of one’s church. I have known men who grew large churches (in one case, a cynic might call it an empire) who later turned out to have been indulging in the most gross, heinous, and vile sins for years and years. I have also known devout, godly men who ministered in near poverty serving small churches (in some cases, ungrateful churches who treated them like hired men).
Sometimes it pleases God for whatever reason to grow a certain church in a certain place at a certain time, for his glory, not ours.
The basis on which a servant will be judged is his faithfulness to what God has given him (Matt. 24:45-51; 1 Cor. 4:2; Rev. 2:10). Not how big or how little his church is.
Why then be holy?
You should be holy because God commands it (Lev. 11:44; 1 Pet. 1:15-16) and because you will be judged one day by the One who demands it (Rom. 14:12; 2 Cor. 5:10) and who is himself holy (Exod. 15:11; Lev. 19:2; 20:26; 21:8; Ps. 22:3; 99:9; Isa. 6:3).
And that is reason enough.