High-altitude platform stations (HAPS) enable large-scale federated learning (FL) in non-terrestrial networks (NTN) by providing wide-area coverage and predominantly line-of-sight (LoS) connectivity to many ground users. However, practical deployments face heterogeneous and non-independently and identically distributed (non-IID) client data, which degrades accuracy and slows convergence. We propose a weighted attribute-based client selection strategy that leverages server-side indicators: historical traffic behavior, instantaneous channel quality, computational capability, and prior-round learning contribution. At each round, the HAPS computes a composite score and selects the top clients, while adapting attribute weights online based on their correlation with validation-loss improvement. We further provide theoretical justification that traffic-derived uniformity can serve as a proxy for latent data heterogeneity, enabling selection of client subsets with reduced expected non-IIDness. Simulations demonstrate improved test accuracy, faster convergence, and lower training loss compared with random, resource-only, and single-attribute baselines.