We were a hot startup and, long story short, we reached to be a slow-growth (mature?) company being more or less #1 in our industry.

However there are workers which are leaving from our company. Asking them in head-to-head talks "why?" they answered (almost) invariably "because I want more money". 

However, in the exit questionnaires only 10% talk about a bigger wage. The others invoke other reasons, the most frequent ones being the lack of future enhancement of their career and/or the capping risk (they feel like they will be in a pigeonhole - something like that).

What can we do in order to retain the good people?